Net 6820 customize mesh gateway limits

[sarahalsmiller]

Description

Allow users of Mesh Gateway to add custom circuit breakers to the mesh gateway XDS config by using a ServiceDefaults object named MeshGateway

• We based this implementation on the first option described in this comment since we didn't want to blow out the scope of this story by adding a new config object, even though we believe that to be the better long term strategy. Configurable limits/circuit breakers on mesh-gateways #13589 (comment)

Testing & Reproduction steps

• Using a built image, tested creating a mesh gateway to make sure the snapshot was updated appropriately
• Added a new golden file test to cover adding XDS

Links

PR Checklist

• [ X ] updated test coverage
• external facing docs updated
• appropriate backport labels added
• not a security concern

[nathancoleman]

Nice work!

I do think creating a mesh-gateway configuration entry would be better since we're only really adding support for a specific field inside service-defaults to the mesh gateway; however, like you said, now isn't the time for that.

[NiniOak]

I think this looks good so I'm going to approve. You can wait for approval from others who have more context on this feature.

[nathancoleman]

LGTM otherwise