[Fleet] Add support for Elasticsearch privileges in data stream manifest

[axw]

Per elastic/package-spec#203 (comment), update Fleet/EPM to take Elasticsearch privileges defined in data stream manifests into account when generating agent API Keys.

When taking privileges from a package's data stream manifest, Fleet must ensure those privileges are restricted to the following set: auto_configure, create_doc, maintenance, monitor, read, read_cross_cluster. If any other privileges are defined, they should either be ignored or lead to an error.

The default privileges should remain as: auto_configure, create_doc.

[elasticmachine]

Pinging @elastic/fleet (Team:Fleet)

[elasticmachine]

Pinging @elastic/fleet (Feature:EPM)

[hop-dev]

Useful links:

PR for adding the field to APM: https://github.com/elastic/apm-server/pull/6139/files
PR for adding the field to the registry: https://github.com/elastic/package-registry/pull/713/files