x/build: build infrastructure secrets should be stored in a single secure location.

[cagedmantis]

Secrets required by services in the build repository do not currently have a canonical storage location. The secrets should also be encrypted and stored in a secure location which has a clear audit log of access and changes made to the secrets. We should explore the possible options for secrets management.

@toothrot @dmitshur @FiloSottile

[gopherbot]

Change https://golang.org/cl/217340 mentions this issue: internal/secret: add secret management package

[gopherbot]

Change https://golang.org/cl/219879 mentions this issue: cmd/gitmirror: migrate secrets to secret manager

[gopherbot]

Change https://golang.org/cl/219939 mentions this issue: cmd/gopherbot: migrate secrets to secret manager

[gopherbot]

Change https://golang.org/cl/222066 mentions this issue: cmd/gerritbot: migrate secrets to secret manager

[gopherbot]

Change https://golang.org/cl/222097 mentions this issue: internal/secret: add secret names for common secrets

[gopherbot]

Change https://golang.org/cl/222177 mentions this issue: cmd/pubsubhelper: migrate secrets to secret manager

[gopherbot]

Change https://golang.org/cl/222665 mentions this issue: internal/secret: upgrade secret manager client to v1