[Ingest] Support agent config versioning

[nchaulet]

Description

We should allow versioning of AgentConfigs.

The version should be visible in fleet UIs, and send to the agent during checkin, also agents could report the config version they are running when they checkin.

Data model update

We should use _version of saved object,
Every-time we modify a datasource we should upgrade AgentConfig _version

Edit: as of ES 7.0+, _version is no longer a simple incremented number. Therefore we should implement our own revision property as a custom saved object attribute.

API update

• We should add a revision property to agent configs and datasources. When a datasource is created/updated/removed, its associated agent config revision should also be bumped. Done in [Ingest] Add revision to agent configs & data sources #59848
• Add config_revision property to agents and make sure agent sends and update this information on ack of config change.

UI update

• Add revision number to agent config table. Done in [Ingest] Add revision to agent configs & data sources #59848
• Add revision number to Fleet agents table. Done in [Fleet] Add config revision to fleet agents #60292

[elasticmachine]

Pinging @elastic/ingest (Feature:Fleet)

[jen-huang]

We should allow user to get older versions of agent, and datasources.

I would skip this for now as the _version prop that we get from saved objects for free is from the same ES property for all documents. However ES does not allow retrieval of previous versions.