Support apply: false option for register_provider

[timriley]

When apply: false is given, a provider's registrations will not automatically be merged with the target container.

This would allow an external provider source to be used, but give the user full control over which registrations from the provider source they want to bring into their target container (e.g. they could choose to carry over only a subset, and possibly rename the keys at the same time).

To support this working fully, we'd need to provide an easy API for merging registrations, ensuring all options from the original registration (block, other keyword options) are carried over.