Add support for overriding the selected provider

[mjcheetham]

Feature description

In GCM Windows it is possible to set the chosen authority to any of the available options, rather than the 'auto' feature which uses the standard input dictionary from Git to determine the correct one.

This is a useful feature to be able to specify and support GitHub Enterprise customers who wish to have their servers detected as GitHub and not basic/services by the generic provider.

The existing configuration for GCM Windows users is...

[credential "mygheserver.com"]
    authority = GitHub

We should either take the same configuration values, or create our own key credential.host/provider whilst taking into account the legacy 'authority' values.

The HostProviderRegistry would be the appropriate component to perform this check and lookup.