[DOCS] Add openAPI specification for APM agent configuration endpoints

[lcawl]

Relates to #12903

This PR updates the OpenAPI specification to include details from https://www.elastic.co/guide/en/observability/current/apm-api-config.html

Related to the outstanding issue #12904, this PR re-adds the original file from #12862

[mergify]

This pull request does not have a backport label. Could you fix it @lcawl? 🙏
To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-7.17 is the label to automatically backport to the 7.17 branch.
• backport-8./d is the label to automatically backport to the 8./d branch. /d is the digit.

NOTE: backport-skip has been added to this pull request.

[carsonip]

I thought rum agentcfg endpoint is almost identical to the regular one, with the exception of auth check and the actual endpoint

[carsonip]

And thanks for translating it from docs to openAPI spec. It really shows how much is missing from the existing docs.

[lcawl]

And thanks for translating it from docs to openAPI spec. It really shows how much is missing from the existing docs.

I wonder if you'd be willing to accept the document in this current state so we have minimal parity with the existing docs. Then I can finish doing the rest of the endpoints and circle back to help add more descriptions and missing values. We want to ensure it's at least as good as the existing docs before we start migrating the rest of the Observabilty docs to the new docs system.

[carsonip]

let's merge on parity and revisit these suggestions later.