FR: Support dual region buckets.

The method for creating a bucket should accept a string with 2 regions concatenated with a '+' (e.g. US-EAST1+US-WEST1).

System tests:

• Verify a dual region bucket with specified regions can be created
• Verify a dual region bucket's location metadata is the specified dual region provided
• Verify a dual region bucket's locationType metadata is dual-region

[Internal] For details and coordination: go/gcs-dpe-dual-region-milestones