[APM] Fleet: Introduce API for uploading source maps for RUM

[axw]

APM Server has an API endpoint for users to upload source maps for RUM: https://www.elastic.co/guide/en/apm/server/current/sourcemap-api.html. Users upload source maps using this API, and then APM Server fetches the source maps back out of Elasticsearch when RUM events are received, in order to map from generated back to source locations in stacktrace frames.

As part of the move to Fleet we will need to move away from fetching source maps directly from (and indexing source maps into) Elasticsearch, as the privileges APM Server is given do not cover this.

I propose that we introduce an API endpoint to Kibana that is roughly equivalent: users will upload source maps to Kibana (e.g. /api/apm/sourcemaps), and then:

• the source map will be recorded as a Fleet Artifact
• a reference to the source map will be injected into APM Fleet policies, including: service name, service version, bundle filepath, artifact download URL

After the server receives the Fleet policy update, it will fetch the source map from Fleet Server, using the given artifact download URL. See elastic/apm-server#5002.

Although the current API is probably not ideal, I think we should make the API in Kibana as close to it as possible. This will simplify users' migration of uploading from APM Server to Kibana.

I've written some horrible POC code for reference: axw@e529752

[elasticmachine]

Pinging @elastic/apm-ui (Team:apm)