[Asset Manager] Add getServices to public client

[jasonrhodes]

Acceptance Criteria

• assetsClient available to dependees, on server and browser
• assetsClient.getServices method exists (accepting the same parameters as the associated REST API) on server and public clients
• Plugin docs updated to reflect new API availability
• options.parent that exists now should be solidified for use with signals and assets source consistently

[elasticmachine]

Pinging @elastic/observability-asset-management (Team:Observed Asset Management)

[jasonrhodes]

@klacabane I'm looking at the server client + REST API and they both accept an options.parent value, but the way the queries are designed, it looks like you'd need to pass a fully-formed EAN value if we're reading from assets, whereas you'd pass just a hostname if we're querying from signals.

I'm considering accepting only an EAN there and then parsing it in the server method and attempting to construct relevant signals-based queries based on the kind, what do you think? Did you attempt that and bail or just go for what worked well enough for now? (Either one fine!)

[klacabane]

Did you attempt that and bail or just go for what worked well enough for now?

I may have only considered a world where you use assets or signals, so consumers provide the appropriate value depending on the active mode. Only accepting ean sounds good to me, the format can be parsed to provide filters in both mode