[ConnectorsV2] Allow specifying custom global headers for a single file connector type.

[ymao1]

Summary

Updates single file connector auth spec to allow specifying custom headers that will be added to the configured axios instance for the connector type.

For our existing connectors, we typically save any header values in the connector saved object as part of the config. For the single file connectors, I thought it would be more dynamic to add it to the ActionType and read it at execution time. This means that if a connector spec is updated with a new header value, existing connectors of that type (stored as saved objects) would use the updated headers the next time they're executed.

[elasticmachine]

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

[elasticmachine]

💚 Build Succeeded

• Buildkite Build
• Commit: 27c5ca4

Metrics [docs]

Public APIs missing comments

Total count of every public API that lacks a comment. Target amount is 0. Run node scripts/build_api_docs --plugin [yourplugin] --stats comments for more detailed information.

id	before	after	diff
actions	373	375	+2

Async chunks

Total size of all lazy-loaded chunks that will be downloaded as the user navigates the app

id	before	after	diff
stackConnectors	826.5KB	826.5KB	+35.0B

Unknown metric groups

API count

id	before	after	diff
actions	379	381	+2

cc @ymao1