[keyserver][lib] Introduce version endpoint

Summary:
This endpoint returns the current version of the keyserver. Its initial use case will be simply to check if a keyserver exists at this URL.

More context in ENG-3952.

Depends on D8189

Test Plan:
Tested in combination with the rest of the stack as follows:

1. Override defaultURLPrefix in native/utils/url-utils.js to return some random string. This makes sure that the "default" of Ashoat's keyserver does not work, and makes it so the registration flow only works if the user selects a custom keyserver and all of the server calls in the registration flow use that custom keyserver
2. Test the registration flow end-to-end for an Ethereum account
3. Test the registration flow end-to-end for a username/password account
4. Test the registration flow end-to-end for a username/password account with a custom avatar that requires an image upload
5. Make sure that for all registrations, the correct avatar appears in the ChatThreadList when the RegistrationNavigator dismisses

Reviewers: michal, atul

Reviewed By: atul

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D8190