[Actions] Expose an API UI client from the start method of the actions plugin

[cnasikas]

There are plugins that use the Actions API directly in the UI. We should expose an API UI client from the start method of the actions plugin (public) for them to consume. This way we can do changes in the API without affecting other teams. This will also be helpful when versioning our endpoints.

APIs to expose:

• Get connectors
• Get action types

[elasticmachine]

Pinging @elastic/response-ops (Team:ResponseOps)

[pmuellr]

Feels like this would be a good fit for alerting as well. Not being familiar with what the client alerting plugin returns, I took a peek - and it's just navigation stuff. So we'd probably do both.

I wonder if this will be a lot easier with the open API stuff - maybe we could generate a client? Actually, I think that's a common use-case for Open API ... :-)

[cnasikas]

Good point Patrick regarding the alerting plugin. We should do the same. If we could generate the client from the Open API it will be amazing!!

cc @XavierM