[Spanner Backup & Restore] Provide ability to run Dataflow in another project to prevent accidental writing on-top of Spanner

[EricBeach]

Enable the Dataflow job performing the backup or restore to run in a different project than the Spaner database. This would allow more detailed ACLs and reduce risk of a data corruption issue.

[nagarkar]

Eric, can be done by external contributors (label it with 'help wanted' or 'good first issue' if appropriate?)

[EricBeach]

Yes