Input configs from units should hard-code shipper connection settings

[fearful-symmetry]

CC @cmacknz
This is similar to #1729

With V2, the shipper is getting a copy of all the inputs for the purpose of configuring GRPC. Right now the logic for this lives in injectShipperConn in pkg/component/runtime/manager_shipper.go. Considering that the shipper needs to make use of that config logic, the struct should be defined in a single place where it can be seen and exported. Ideally, this would be a set of fields in proto.UnitExpectedConfig{}.

[cmacknz]

We spoke about this today.

My opinion is that the problem of defining schemas for configuration sent over the agent control protocol and validating received configuration against it is a general one that affects every process running under agent. I would like to see a solution to this, but the problem is big enough that we shouldn't make solving it a requirement of the shipper project.

We discussed some more targeted ways to improve the way configuration is handled in the shipper without tackling a larger and more general problem:

• @faec has some ideas around a callback driven mechanism for detecting unknown and missing fields.
• @leehinman suggested incorporating something similar to the elastic-package linter for integrations https://github.com/elastic/package-spec/blob/main/spec/integration/manifest.spec.yml

[pierrehilbert]

@cmacknz / @leehinman could we close this one with the new shipper plan?

[cmacknz]

This can stay open since it affects the Elastic Agent itself, not the shipper. I have this categorized as technical debt, so we don't need it for any particular release.

[jlind23]

The shipper project has been revisited hence closing this as outdated/not relevant.