Feat: entry points with override methods

[tiran]

The new entry point fromager.override_methods lists all fromager override methods by name and their default implementation. This allows consumers to get a list of known override methods and compare function signatures.

Also address some typing issues in override methods and helpers.

[dhellmann]

I like this. We could document using entry_point_inspector for listing them.

This is relevant to #196, too, @shubhbapna