The command validates a given OIDC configuration, either from a referenced
Secret or from CLI flags. This will help users debug issues with Weave GitOps
OIDC configuration as well as provide a way to validate a configuration before
putting it on a cluster.

The command consumes OIDC configuration from CLI flags or from a Secret on a
cluster and sends the user through an OIDC authorization code flow. If it
succeeds, the username and groups claims are logged to stdout. The command
validates the Secret for missing fields and also prints all errors returned
from the OIDC provider so that users know what went wrong.

This will work out of the box with OIDC providers given they are configured to
accept "http://localhost:9876" as a redirect URI.

This commit also aligns the existing `--claim-username` flag with the
K8s one and renames it to `--username-claim`.

The generated files have been updated to reflect the updated version
of the buf command.

golang.org/x/oauth2 changed its error handling in v0.8.0
(https://go-review.googlesource.com/c/oauth2/+/451076) to actually
parse error responses and return the error code in the returned error
variable.