feat(spanner): Auto-generated executor framework proto changes (#8713)

* feat(spanner): add autogenerated protos for executor framework

* feat(spanner): update readme

* feat(spanner): update directory

* feat(spanner): update directory

* feat(spanner): update directory

* feat(spanner): update directory

Author: harshachinta

spanner/test/cloudexecutor/proto/README.md

@@ -0,0 +1,7 @@
+# Regenerating protos
+
+Cloud Spanner Executor Framework - To generate code manually for cloud_executor.proto file using protoc, run the command below.
+```
+cd spanner/test/cloudexecutor/proto
+protoc --go_out=plugins=grpc:. -I=<local path to googleapis> -I=./ cloud_executor.proto
+```