Support providing OAuth 2 bearer tokens directly to the plugin

[jlawrienyt]

This is to support a use case where a Vault plugin is being used to provide secrets to Drone, and using the Google Cloud Secrets Engine which provides either an OAuth 2 Token or a base-64 encoded service account json key, neither of which will currently work with this plugin.

I'm happy to update the plugin docs as well, though I'm not sure where the source for those is.

[techknowlogick]

The plugin docs can be found here: https://github.com/drone/drone-plugin-index

[jlawrienyt]

The build failures are due to docker.com rate limiting:

Step 1/12 : FROM alpine:3.12 as alpine
toomanyrequests: You have reached your pull rate limit. You may increase the limit by authenticating and upgrading: https://www.docker.com/increase-rate-limit
exit status 1

Any thoughts on how to get the build working? I see some discussion over in discourse, but I'm not sure how it applies to builds running at cloud.drone.io.

[lafriks]

Not much can be done sadly :/

[lafriks]

Ok, restarted job and got lucky 😁

[tonglil]

The suggestion here was to remove the dryrun step.

[jlawrienyt]

I've created a draft PR in the plugin docs repo documenting the update. Anything else that needs to be done here before it can be merged?