chore(spanner): vendor grpc-gcp sources into google-cloud-spanner #17574

Job	Run time
package_name_check	1m 4s
generate-readme	1m 6s
group_id_check_for_maps_libraries	8m 7s
owlbot-yaml	1m 2s
gapic-bom	1m 41s
owlbot-py	1m 11s
root-pom	1m 57s
gitignore_correctness	1m 5s
	17m 13s

Provide feedback