[vtadmin-api] Add cluster field to vtadmin-api's /api/gates response

[doeg]

Signed-off-by: Sara Bee 855595+doeg@users.noreply.github.com

Description

Updates vtadmin-api's /api/gates endpoint to return cluster information.

Previously:

{"result":{"gates":[{"hostname":"127.0.0.1:15991"}]},"ok":true}

Currently:

{"result":{"gates":[{"hostname":"127.0.0.1:15991","cluster":{"id":"id1","name":"cluster1"}}]},"ok":true}

Related Issue(s)

N/A

Checklist

• Should this PR be backported?
• Tests were added or are not required
• Documentation was added or is not required

Deployment Notes

N/A

Impacted Areas in Vitess

Components that this PR will affect:

• Query Serving
• VReplication
• Cluster Management
• Build/CI
• VTAdmin

[doeg]

I defined these expectedCluster*Gates slices rather than adding the Cluster field to the existing cluster*Gates slice. Prior, when I added the Cluster to a cluster*Gates slice, fakedisco added them as-is, meaning the test passed without exercising the new codepath.