[Gateway] Add support for gateway token renewal

[kevin-bates]

This pull request adds the ability for operators to configure a callable to perform authorization token renewal for the token used to access a Gateway server.

The callable class is specified via the GatewayClient.gateway_token_renewer_class configurable trait (or env JUPYTER_GATEWAY_TOKEN_RENEWER_CLASS). The specified class name, once instantiated, must be a subclass (i.e., instance of) jupyter_server.gateway.gateway_client.GatewayTokenRenewerBase, which is an abstract base class requiring the implementation of a single method - get_token():

    @abstractmethod
    def get_token(
        self,
        auth_header_key: str,
        auth_scheme: Union[str, None],
        auth_token: str,
        **kwargs: Any,
    ) -> str:
        """
        Given the current authorization scheme and token, this method returns
        a (potentially) renewed token for use against the Gateway server.
        """
        pass

GatewayTokenRenewerBase derives from LoggingConfigurable and, as a result, can support its own configurable traits that may be necessary to perform token renewal. In addition, the instance's self.parent attribute contains the current instance of GatewayClient, where other attributes are accessible. (Note: auth_header_key, auth_scheme, and auth_token also reside on GatewayClient, but are provided as parameters for convenience.)

The gateway tests have been extended to demonstrate the configuration of a custom token renewer with configurable traits.

[codecov-commenter]

Codecov Report

Base: 72.52% // Head: 72.59% // Increases project coverage by +0.06% 🎉

Coverage data is based on head (86a60fa) compared to base (32e0919).
Patch coverage: 82.25% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #985      +/-   ##
==========================================
+ Coverage   72.52%   72.59%   +0.06%     
==========================================
  Files          64       64              
  Lines        8205     8239      +34     
  Branches     1375     1378       +3     
==========================================
+ Hits         5951     5981      +30     
- Misses       1840     1844       +4     
  Partials      414      414              

Impacted Files	Coverage Δ
jupyter_server/gateway/gateway_client.py	78.18% <82.25%> (+1.15%)	⬆️

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[blink1073]

Thank you!

[kevin-bates]

Thank you Steve!