[eem] readSourceDefinitions to accept multiple types as input

[klacabane]

See #202298 (comment) for context

Api consumers may need to read n type sources in a path which results in n requests executed. We should update the readSourceDefinitions api to accept multiple types so we only have to execute a single request

[elasticmachine]

Pinging @elastic/obs-entities (Team:obs-entities)

[miltonhultgren]

I wonder if perhaps we should avoid exposing sources directly like this?

Or rather, perhaps when reading the type, we automatically also include all the sources? Or that the type endpoint takes an option for that behaviour and a filter for which types you care about?

I think that would align better with what consumers are wanting to achieve, since from their perspective it's all about the type (the source being a bit of a sub detail that we force on them as a separate API call).

WDYT?