GCStorage - Handle extension headers on signUrl generation

[frantello]

More advance configuration and validations of sign URLs require extension headers [https://cloud.google.com/storage/docs/xml-api/reference-headers](extension headers).

Opened this ticket to track my commits.

I'm working on a pull request that will coming soon.

[yihanzhen]

@frantello it seems the PR this issue tracks is merged. Can I close it now?