Add a downstream check for spring-cloud-gcp

[blakeli0]

spring-cloud-gcp is dependent on gapic-generator-java-bom, we should make sure that any changes in this repo should not break spring-cloud-gcp by adding a downstream check. This happened in the past that a change in TestProtoLoader break SpringCodeGen, see https://github.com/GoogleCloudPlatform/spring-cloud-gcp/pull/2570.

[diegomarquezp]

@blakeli0 Do we only want to check autoconfig generation?

[diegomarquezp]

From discussion with @blakeli0, let's limit the scope to updating the generator