processor: add datadogprocessor

[gbbr]

This change adds the processor described in #15689. It is the initial PR containing the structure and implementation as per contribution guidelines.

I added the implementation too, because it is a trivial, small one. If necessary, I am happy to split more. Besides the processor/datadogprocessor/agent.go file, the rest is boilerplate.

Requires #16852

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TestLoadInvalidConfig_NoScrapers ❌ failed 12 times in 12 runs (100% fail rate).

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[gbbr]

Addressed.

[mx-psi]

I left some more comments, also please take a look at https://github.com/open-telemetry/opentelemetry-collector-contrib/pull/16853/files#r1045930895

[gbbr]

I hope I didn't miss anything...

[mx-psi]

Skipping changelog, will add when we add to components

[mx-psi]

Looks like there are some lint failures after bumping to v0.67.0, @gbbr can you have a look?

[gbbr]

Oops! Should be ok now.

[mx-psi]

I think the only remaining issue to be addressed is to run make generate-gh-issue-templates and commit the changes so that the checks job passes.