[OAS] Add serverless connector APIs

[lcawl]

Summary

Relates to #152378

This PR creates a separate openAPI specification for the connector APIs that are available in Elastic Serverless.
From my tests, at this time these APIs don't differ from their classic counterparts except they:

• Support only API key authentication
• Do not support space ID path parameters

Since I do not yet know what privileges or roles are required to run these APIs, I have omitted that information for now.

I've also made the following fixes and changes:

• Clarified that the config is not visible when in the response when it's a preconfigured connector.
• Added reference_by_count to the response schema with a note in its description that it's applicable only to the "get all connectors" APIs.
• Added the missing generativeAI feature value.

[apmmachine]

🤖 GitHub comments

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Just comment with:

• /oblt-deploy : Deploy a Kibana instance using the Observability test environments.
• /oblt-deploy-serverless : Deploy a serverless Kibana instance using the Observability test environments.
• run elasticsearch-ci/docs : Re-trigger the docs validation. (use unformatted text in the comment!)

[elasticmachine]

Pinging @elastic/response-ops (Team:ResponseOps)

[kibana-ci]

💛 Build succeeded, but was flaky

• Buildkite Build
• Commit: a735f2e

Failed CI Steps

• Security Solution Cypress Tests #1
• Serverless Osquery Cypress Tests #1
• Serverless Osquery Cypress Tests #6

Test Failures

• [job] [logs] Security Solution Cypress Tests #1 / Enrichment Custom query rule from new risk scores Should has enrichment fields from legacy risk Should has enrichment fields from legacy risk

Metrics [docs]

✅ unchanged

History

• 💛 Build #167849 was flaky f262e027eb2d0b2d77cbc87110746f4d351cd167
• 💔 Build #167567 failed 7e2d59044afa8f9e45d69363dcbbf91998f2e16a
• 💔 Build #167558 failed 82d454c1b276e82bc8af56a6064df2e31355fcd9
• 💔 Build #167532 failed 6c279c5b827e2a461bc964a23df27f42897d3eb7
• 💛 Build #167482 was flaky c8b63fb4b55511126ceaf62794bb2e7678a44773
• 💛 Build #167174 was flaky ad080816c21445f4d61d3805f97433e8d64caee9

To update your PR or re-run it, just comment with:
@elasticmachine merge upstream

[ymao1]

LGTM