Fixes https://github.com/go-gitea/gitea/issues/39925
RFC 6749 §2.3.1 has clients form-urlencode the client id and secret
before building the HTTP Basic header. `openid-client` with
`client_secret_basic` does this and sends `-` as `%2D` and `_` as `%5F`.
The token and introspection endpoints used the raw header values, so
these clients failed with `invalid_client`. Both endpoints now decode
the values with `url.QueryUnescape` before the client lookup, the secret
check and the body consistency check, and reject a value that does not
decode. Plain HTTP Basic parsing in `httpauth` is unchanged, since the
encoding only applies to OAuth client authentication.
Clients that don't encode are unaffected, since no client id or secret
Gitea has ever issued contains `+` or `%`.
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Co-authored-by: silverwind <me@silverwind.io>
