Fix snapshot.get validation errors

[sethmlarson]

• Adds the missing index_names parameter which is a part of the JSON spec
• Only remaining validation errors will be fixed by REST spec for get-snapshots API missing pagination-related parameters elasticsearch#88620

[sethmlarson]

Since this includes a breaking change in making the responses' indices property optional I won't backport this one automatically.

[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	🟢	26/26	26/26
snapshot.create	🟢	26/26	26/26
snapshot.delete_repository	🟢	10/10	10/10
snapshot.delete	🟢	20/20	20/20
snapshot.get_repository	🟢	19/19	19/19
snapshot.get	🟢	13/13	13/13
snapshot.repository_analyze	🟠	Missing type	Missing type
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.

[stevejgordon]

LGTM