[Dashboards] Embeddables schema registry for Dashboard API validation

[nickpeihl]

Follow up to #192618

Implement a registry for embeddables to optionally register their configuration schemas for stricter validation and a better API schema definitions for the public Dashboards API. If a schema is not registered for an embeddable, the configuration will not be validated and the OpenAPI specification for that embeddable type will not be defined.

[elasticmachine]

Pinging @elastic/kibana-presentation (Team:Presentation)

[thomasneirynck]

offline feedback from @lukeelmers has raised some open questions around the introduction of validation after the release of MVP.

Questions to answer:

• When somebody has included invalid attributes, before an embeddable-type had validation, what would happen after validation is introduced for that type?
  • errors
  • warnings
  • ignore invalid
• What is the API-surface for a user to enable/disable validation?
  • enabled by default and not user-facing
  • opt-in/opt-out (e.g. ?strict=[true|false])
• Should moving to GA be delayed until validation is in place? The answer here depends on some of the decisions made above.
• Should moving to GA be dependent on having validation on all embedabbles?