ProxyOAuthServerProvider: redirect_uri missing in token request

[maschnetwork]

Describe the bug

When calling the /token endpoint in exchangeAuthorizationCode the ProxyOAuthServerProvider does not send redirect_uri - which is required according to the spec. This leads to token endpoints returning 400/500 when (for example) proxying to Amazon Cognito.

To Reproduce
Steps to reproduce the behavior:

1. Leverage ProxyOAuthServerProvider as described in the docs
2. Return 401 to trigger Authorization Flow
3. /authorize works
4. /token fails

Expected behavior

The redirect_uri that was used during /authorize should be sent to the token endpoint.

[matt-vendia]

Hit the same issue. Required by AWS Cognito.