Incompatibility with bazel credential helper?

[jayconrod]

Hello! I'm one of the authors of engflow_auth, a Bazel credential helper that works with EngFlow.

One of our users asked if they can use engflow_auth together with reclient. Sadly, it looks like this doesn't work. It looks like this repo supports a credential helper in credshelper.go, but the protocol is not the same.

1. Would you consider implementing the Bazel credential helper protocol? It would be nice to avoid incompatibility here. I'm happy to send a PR if that would be useful.
2. If not, is the current credential helper protocol stable? Can it be documented?
3. Is there any way to use the credential helper without OAuth2? Our credential helper does use OAuth2 to obtain a token in the background, but Bazel isn't aware of that, and it isn't surfaced in the credential helper protocol. We'd like to set HTTP headers to opaque strings.