feat: allow using custom domain name when creating signed resources

[AVVS]

No description provided.

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[AVVS]

I signed it!

[googlebot]

CLAs look good, thanks!

[jgeewax]

Thanks @AVVS ! Looks like there's some trailing whitespace and a line over 80 characters, but if those are fixed this LGTM!

[coveralls]

Coverage remained the same at 100.0% when pulling 007a9c5 on AVVS:patch-1 into 24810b9 on GoogleCloudPlatform:master.

[stephenplusplus]

Thanks, @AVVS! I made a few trivial changes and merged them in via 0a6da60