deferred actions: implement latest provider protocol proposals

[liamcervante]

This PR implements the latest proposal for the changes to the provider protocol with support for deferred actions.

We're moving the deferral_allowed field into a common ClientCapabilities message type to mirror the ServerCapabilities struct. This will make further changes of this type easier to implement in the future, as feature triggers like this can be added to the capabilities message once instead of to all the RPC requests directly.

In addition, adding it to the ConfigureProvider RPC. The ConfigureProvider response is not changing, so this is just an FYI to help the providers prepare for future deferral requests.

There is no behaviour change in this PR, the only difference is in the format we are sending the deferral_allowed field to the providers.

@DanielMSchmidt, this may break some of your in progress PRs but the fixes should be relatively trivial. Just adding in the extra ClientCapabilities wrapper.

[austinvalle]

Protocol changes look good to me, thanks! 🎸

[bflad]

Looks good to me 🚀

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