feat: Provide arguments to service via pebble run --args

[rebornplusplus]

The goal is to introduce the concept of optional and overrideable
arguments for a Pebble service, but we must ensure that the layering
properties of Pebble are preserved and respected. The proposed syntax
re-uses therefore the existing service “command” attribute:

services:
    myservice:
        command: myservice --db=/var/db [ --quiet ]

The CLI is extended to support replacing the default command
arguments defined in the plan, with the ones given at execution time.
Inspired by the -exec syntax of the find command-line utility,
we propose adding a new option to pebble run, called --args.

Usage:
    pebble run --args SERVICE [ARGS ...] [TERMINATOR]

SERVICE:      required name of the Pebble service that will
              receive the arguments;
[ARGS ...]:   arguments to be passed to the Pebble service named
              “SERVICE”;
[TERMINATOR]: a “;” symbol (backslash-escaped if using the 
              shell) to indicate that whatever follows is not 
              an argument for “SERVICE”
                  - this symbol is only required whenever other 
                    Pebble-specific options and/or commands are 
                    given after --args, otherwise can be omitted.

With this, we can provide additional arguments to multiple services
defined in the plan, whether they hold default arguments or not.

Examples:

Start the Pebble server and only pass some arguments to “svc”:

pebble run --args svc --verbose --foo "multi str arg"

Start the Pebble server with all services on hold, and give some
args to “svc”:

pebble run --args svc --verbose --foo "multi str arg" \; --hold

Start the Pebble server and pass arguments to multiple services:

pebble run --args svc1 --verbose --foo "multi str arg" \; \
           --args svc2 --somearg 1 --payload '{"foo": "bar"}'

[rebornplusplus]

Like @woky mentioned in #188,
I have included https://github.com/rebornplusplus/go-flags
which supports terminated options like --args here. There is a PR
against upstream jessevdk/go-flags#395. But
I am not sure when it will get any response from the maintainer.
Should we consider forking go-flags and use that for now?

The conflicts in this PR come due to the usage of fork. (rebased)
Also, I have a few scattered commits here. I intend to squash
them before they are okay to go in.

[benhoyt]

This looks like a good first pass, thanks! I've left a bunch of comments/suggestions, with two main structural concerns:

1. I think we can simplify significantly just by avoiding the API/client stuff, and pass --args to the daemon (and from there, to the service manager) directly.
2. I think we should separate out the CmdArgs handling from the plan, as it's not really related to the plan, but a separate override of the plan. I think it'll clean it up and probably simplify to extract this.

[rebornplusplus]

Hi @benhoyt, I tried to address your comments (they were super helpful!) in de12f37. Please take a look and let me know what you think. Thank you!

[benhoyt]

Looking much better, thanks! I'm approving but with a few minor suggestions for some text clarity and a simplification about how you're passing the args down to overlord and service manager.

[benhoyt]

Excellent, thanks for that -- looks good to me now.

[niemeyer]

Thanks for this Rafid. It's looking very promising.

Here is a first pass:

[rebornplusplus]

Hi @niemeyer, thanks for your insightful comments. I have added a commit addressing them. Will you please take another look?

Additionally, the new commit will require the PR canonical/x-go#19 to go in. Will you please take a look there as well? That one is awaiting one more approval.

[rebornplusplus]

Also, I have added a PR in canonical/go-flags#2 which will be needed for this PR as well. It will be really helpful if you can check it too.

[woky]

Nice. LGTM.

[niemeyer]

Thanks for the changes, Rafid, this is much closer.

Here is a second pass:

[rebornplusplus]

Note about the failing "Tests / Root Tests": It passes in my GitHub repo and local workstation. https://github.com/rebornplusplus/pebble/actions/runs/4675467143 (successful GH action for the same set of commits)

[woky]

@niemeyer @rebornplusplus @cjdcordeiro The test failure was not fault of this PR. Now all checks are passing.

LGTM!

[benhoyt]

Thanks! Two small requests: 1) label, and 2) test SetServiceArgs

[niemeyer]

Looking good now, thank you.

Probably the last pass:

[rebornplusplus]

Hiya @niemeyer. Thanks for the reviews. I have updated the requested changes in the recent commits. I have updated the version of the canonical/x-go module as well. Tests are passing.
There have been a bunch of commits in this PR and when the PR is ready to be merged, I would like to squash the commits into one before merging.

[woky]

LGTM

[niemeyer]

Thank you! Great to see this going in.