Refactor LoginFlowManager interactions with GlobusApp

[derek-globus]

Made some changes to the way LoginFlowManagers are handled in GlobusApp to improve the native login experience, support confidential clients in UserApps, and make it easier to configure the correct login_flow_manager.

Changelog

• Changed the experimental GlobusApp class in the following ways (:pr:NUMBER):

  • LoginFlowManagers now insert GlobusApp.app_name into any Native
    client login flows as the prefill_named_grant.

  • GlobusAppConfig now accepts a login_redirect_uri parameter to specify
    the redirect URI for a login flow.

    • Invalid when used with a LocalServerLoginFlowManager.

    • Defaults to "https://auth.globus.org/v2/web/auth-code" for native
      client flows. Raises an error if not set for confidential ones.

  • UserApp now allows for the use of Confidential client flows with the use of
    either a LocalServerLoginFlowManager or a configured login_redirect_uri.

  • GlobusAppConfig.login_flow_manager now accepts shorthand string references
    "command-line" to use a CommandLineLoginFlowManager and
    "local-server" to use a LocalServerLoginFlowManager.

  • GlobusAppConfig.login_flow_manager also now accepts a
    LoginFlowManagerProvider, a class with a
    for_globus_app(...) -> LoginFlowManager class method.

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📚 Documentation preview 📚: https://globus-sdk-python--1018.org.readthedocs.build/en/1018/

[ada-globus]

Approved as I think this all functionally looks great. I do have some suggested changes vis-a-vis docs and a couple microscopic nits re: comments.