chore(spanner): add ReplicaSelector interface and impl #17523

Job	Run time
owlbot-py	1m 10s
package_name_check	1m 10s
group_id_check_for_maps_libraries	8m 31s
root-pom	1m 50s
gitignore_correctness	1m 10s
gapic-bom	1m 50s
generate-readme	1m 15s
owlbot-yaml	1m 10s
	18m 6s

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