Pipeline tests

[thampiotr]

PR Description

This demonstrates how we can run the agent programmatically with a fully-fledged config file and some fake source and destination endpoints.

I think that this approach has some benefits:

• It can run quite fast, without delaying the CI pipeline*
• It verifies that components integrate with each other
• will expose a wide range of breaking changes to configs if made by accident or when a dependency is updated
• can be used to check for regressions against some of the most common customer pipelines
• the configs can be used as examples too in the docs, so we can verify that our examples work
• we can debug and step through these with IDE

Some of the cons are:

• Need to add fake implementations of external stuff. Sometimes it's easy, but sometimes it may be harder, e.g. for some exporters.
• it is some extra code, but hopefully copying and tweaking existing examples is simple enough

*this is currently not quite the case for prometheus due to WAL, but we can probably improve it with some effort.

Which issue(s) this PR fixes

Notes to the Reviewer

PR Checklist

• CHANGELOG.md updated
• Documentation added
• Tests updated
• Config converters updated

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