Replace usage of jsonwebtoken for GCS

[Jake-Shadle]

This removes the use of the patched jsonwebtoken

jsonwebtoken = { git = "https://github.com/Jake-Shadle/jsonwebtoken.git", rev = "2f469a61" }

which is still needed due to no release from jsonwebtoken with the relevant changes (last release was May). Using patches are supposed to be temporary, because what ends up happening is that people that do cargo install sccache --features=gcs end up with a broken sccache because the patched version of jsonwebtoken that "works" is not used when installing from crates.io, and for some reason, the prebuilt binaries (at least the musl one) in github releases are also incorrect.

Rather than wait for a release, this change removes the need for jsonwebtoken altogether for gcs caching, by just doing the relevant pieces directly using base64/ring.

• Serializes (JSON + base64) the JWT header (signing algorithm)
• Serializes (JSON + base64) the JWT claim (scope, timestamps, uri etc)
• Signs the JWT payload with the private key from the service account
• Concatenates the JWT payload and the base64 encoded signature

jsonwebtoken is still used in the dist stuff, only required 2 changes due to a field being removed from JWT.

Resolves: #489

[froydnj]

Can you rebase this now that we're resolved the weird failures with jobserver? Thanks!

[froydnj]

Hm, it looks like development is happening on jsonwebtoken...but it doesn't seem to be happening fast enough. We can merge this for now, but I'd love it if/when jsonwebtoken gets updated if this change were reverted.

Can you force-push to squash your commits and preserve bisectability, please?

[Jake-Shadle]

Done!

[froydnj]

Thank you!