Add process based plugins to Smithy build

[mtdowling]

This commit adds the ability to integrate smithy build with process based plugins so that the Smithy CLI will be able to call other CLI tools through a process based plugin model.

To do this, I've introduced the run plugin which takes a command array containing the program to run and arguments to provide. The plugin will open a process and send the serialized Smithy model to the standard input of the process. The process can then perform its logic and write files in the working directory its called with.

A couple things are still missing here that we should figure out later:

1. The only way to see the output of a plugin is using --debug in the CLI. Should there be an output to somehow always show it? That will require some kind of hackery in the CLI I think.
2. There is no way currently to indicate the process is "serial" like how other Smithy Build plugins can. Serial indicates that a plugin requires some kind of global locking (like file locks) and can't be run in parallel with other plugins.

In addition to adding the run plugin, you can also apply the same plugin multiple times in a single projectiony by giving it a custom artifact name using the "plugin-name::artifact-name" syntax.

Example:

{
    "version": "1.0",
    "projections": {
        "source": {
            "plugins": {
                "run::hello": {
                    "command": ["hello.sh", "--arg", "arg-value"]
                }
            }
        }
    }
}

Issue #, if available:

Description of changes:

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[sugmanue]

I left some minor notes.

[sugmanue]

I just noted some duplication, not sure if those are per-design, i.e., part of the tests. I added a comments anyway.