Find user identifier before getting keys

Rebase

Rebase before landing

Rebase, add comments explaining challenge response

Use 'authoritative' where applicable

Rebase

In D11379#329930, @varun wrote:

do we need to change any client code now that some of these RPCs are returning "not found" ?

Some comments/proposals appeared in ENG-7598 that we need to take into account here. Requesting review now to open discussions

Read the issue and seems like it was unrelated to this code.

• Made get_user_identifier() return Option
• Updated callsites to return Status::not_found if missing identifier

Requesting changes to discuss possible usage of BatchGetItem

In D11379#328585, @ashoat wrote:

At a later point we might want to introduce a feature where somebody with an ETH account can still have a username. This would make sense for ETH users that don't have an ENS name.

I wonder if we should future-proof this API now. I guess the best thing to do would be to modify Identity as follows:

message Identity {
  string username = 1;
  optional EthereumIdentity eth_identity = 2;
}

If the user is using their ETH wallet address as a username, then username will just be set to the wallet address.

Rebase before landing

Make types readonly

Address feedback