Add documentation for GET snapshots state query parameter.

[JeremyDahlgren]

Adds documentation for the SnapshotState enum and state query parameter added in elasticsearch PR 128635.

[github-actions]

Following you can find the validation results for the APIs you have changed.

API	Status	Request	Response
snapshot.cleanup_repository	🟢	3/3	3/3
snapshot.clone	🟢	5/5	5/5
snapshot.create_repository	🔴	24/29	29/29
snapshot.create	🟢	30/30	30/30
snapshot.delete_repository	🟢	10/10	10/10
snapshot.delete	🔴	22/24	24/24
snapshot.get_repository	🔴	19/19	7/19
snapshot.get	🔴	15/16	16/16
snapshot.repository_analyze	⚪	Missing test	Missing test
snapshot.repository_verify_integrity	⚪	Missing test	Missing test
snapshot.restore	🟢	5/5	5/5
snapshot.status	🟢	2/2	2/2
snapshot.verify_repository	🟢	2/2	2/2

You can validate these APIs yourself by using the make validate target.

[pquentin]

Thank you! Note that the Elasticsearch specification has multiple uses, and documentation is only one of them. The original goal was to generate Elasticsearch clients, and this is how we generate 7 of our clients now.

[pquentin]

For better user experience, comma-separated values in query parameters are exposed as lists in Elasticsearch clients, so we should allow SnapshotState but also SnapshotState[], as shown below. I had to remove the default value line too because of this error: Error: Default values in unions can only be configured for instance_of types, you are using union_of.

Suggested change

	/**
	* Only return snapshots with a state found in the given comma-separated list of snapshot states.
	* The default is all snapshot states.
	* @server_default All snapshot states
	* @availability stack since=9.1.0
	* @availability serverless
	*/
	state?: SnapshotState
	/**
	* Only return snapshots with a state found in the given comma-separated list of snapshot states.
	* The default is all snapshot states.
	* @availability stack since=9.1.0
	* @availability serverless
	*/
	state?: SnapshotState | SnapshotState[]

With this change, validation passes for snapshot.get, as you will see in the GitHub comment.