[Http/Refit] Add support for custom EndpointOptions to customize endpoint configuration

[nickrandolph]

What would you like to be added:

Ability to register a sub-class of EndpointOptions to allow for additional customization of the httpclient

Why is this needed:

If you want to include an api key in appsettings you currently need to create a separate section and then apply that configuration to the httpclient. A custom EndpointOptions could simply include an additional property that would be available at the point where the httpclient is being created