This allows constructing blob stores with a URL syntax at runtime,
by passing the --blob-service-addr arg.
We probably still want to have some builder pattern here, to allow
additional schemes to be registered.
Change-Id: Ie588ff7a7c6fb64c9474dfbd2e4bc5f168dfd778
Reviewed-on: https://cl.tvl.fyi/c/depot/+/8742
Reviewed-by: tazjin <tazjin@tvl.su>
Tested-by: BuildkiteCI
