Add a bazel integration test

[parthea]

An integration test should be added to ensure a successful build for the following bazel target

gapic-showcase/schema/google/showcase/v1beta1/BUILD.bazel

Lines 29 to 46 in 102c10a

	proto_library(
	name = "showcase_proto",
	srcs = [":compliance.proto", ":echo.proto", ":identity.proto", ":messaging.proto", ":sequence.proto", ":testing.proto" ],
	deps = [
	"@com_google_googleapis//google/api:annotations_proto",
	"@com_google_googleapis//google/api:client_proto",
	"@com_google_googleapis//google/api:field_behavior_proto",
	"@com_google_googleapis//google/api:resource_proto",
	"@com_google_googleapis//google/api:routing_proto",
	"@com_google_googleapis//google/longrunning:operations_proto",
	"@com_google_googleapis//google/rpc:status_proto",
	"@com_google_googleapis//google/rpc:error_details_proto",
	"@com_google_protobuf//:duration_proto",
	"@com_google_protobuf//:empty_proto",
	"@com_google_protobuf//:field_mask_proto",
	"@com_google_protobuf//:timestamp_proto",
	]
	)

See #1475 where the bazel target could not be built at HEAD

[noahdietz]

lmk if you want me to take this, otherwise, assign it to yourself if you are adding it already