rustls auth_provider oauth support

[clux]

You will notice that the initial rustls support from #66 and #114, the oauth2 module was basically commented out leaving gke a bit unusable (or so I think).

Any help porting src/oauth2/mod.rs to rustls would be appreciated. It was mostly ynqa's work from the original source tree, with some help from @davidB recently I think. But anyone feel free to jump on it. I have done the bare minimum to allow some clusters to at least work with rustls.

[NAlexPear]

leaving gke a bit unusable (or so I think)

Just confirmed that this ☝️ is the case while working on a GKE cluster using load_kube_config() and service account credentials. RUST_BACKTRACE shows an unwrap error on one of these lines in the into_token method.

The problem doesn't exist in version 0.6, so sticking with that version for now.

[clux]

Wait, is this the case default features? It should only affect rustls-tls installations.

[davidB]

I don't have GKE cluster, next week-end I'll re-setup a GKE cluster to take a look at this issue, and see If I can help

[NAlexPear]

I was running with the openapi feature only, but if this behavior is surprising, then I'll do my best to get you a more concrete repro later today.

[davidB]

@NAlexPear I was able to reproduce the issue with into_token, it was caused by a 400 response from gke during the refresh (I don't know why), the PR #123 includes display of the response from gke if response is not OK.

[clux]

Have released this PR in 0.25.0 (first rustls support release)

[clux]

This issue should have been closed a while ago.
As it stands the rustls-tls switches everything at the moment, and oauth for gcp is done via tame_oauth now.