[native] Introduce RegistrationContext

Summary:
Explanation for why we need a context is in ENG-4047 and ENG-4043.

This diff just introduces an empty context; later diffs will actually implement functionality.

Test Plan:
The whole stack was tested as follows:

1. Try creating an account with the default avatar
2. Try creating an account with an emoji avatar
3. Try creating an account with an image avatar (that needs to be uploaded)
4. Try creating an Ethereum account with an ENS avatar

I made sure of the following:

1. Loading spinner appeared on the button, not on the avatar, and was present throughout
2. After the RegistrationNavigator is dismissed, the avatar that appears for the user's PRIVATE chat in the ChatThreadList is correct
3. Subsequent delete-account-and-then-register-again worked correctly
4. Various error cases are triggered correctly

Reviewers: atul, ginsu

Reviewed By: atul

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D8148