[OneDiscover][Extension] Allow providing custom ad-hoc data views

[kertal]

📓 Summary:

The contextual awareness framework in Discover should allow providing a custom set of ad-hoc data views in a given context. These ad-hoc dataviews should be selectable in the Data view picker

Note: ideally those ad-hoc data view should have a static id, to make sure shared links are not broken, this is not a functional requirement, but should be noted for consumers.

✔️Acceptance criteria:

• Discover's core should allow the registraton of addtional ad-hoc data views, independend of the internal state.
• These data views should be selectable in the DataView picker
• Discover's contextual awareness framework should be extended by an extension point allowing to register a set of additonal ad-hoc data views
• There should be an example profle making use of this extension point
• Apart from unit tests is should be covered by functional tests using the example profile

[elasticmachine]

Pinging @elastic/kibana-data-discovery (Team:DataDiscovery)