Define a Buildkite pipeline to update the Kibana OpenAPI spec

[tobio]

We want to be able to validate the Kibana OpenAPI spec is valid within the Kibana PR checks. This Buildkite pipeline should:

• Accept a target ref to pull the spec from
• Regenerate the Kibana client from that ref
• Build the provider
• Execute the acceptance tests

There's some things we need to figure out:

• Do we automatically apply valid spec updates to the provider. IMO the pipeline should commit client changes based on spec updates which pass CI directly to main.
• How are valid breaking changes handled. For example, we update a field to accept both a string and an integer. This isn't a breaking change at the API level, but will potentially break the provider CI as the generated client structure will change.
  • If we make this check informational then we lose most of the utility, the first breakage will cause all further Kibana PRs to also fail this check and it will quickly be ignored.
  • One option here is that CI creates a provider PR based on the spec in the Kibana PR. We (i.e the author of the Kibana PR) can then update the provider codebase to handle the changing API, and then merge both provider and Kibana PR's together.