[keyserver][lib] Plumbing for new exact user search endpoint

Summary:
We want to be able to query whether a username exists in order to decide whether to show this screen.

In the case where the ETH wallet already has an account, we'll let them log in directly and (potentially?) skip the avatar-setting step.

This diff:

1. Introduces new types for the input and output of the new endpoint
2. Updates the endpoints types and adds plumbing for the endpoint on the keyserver side
3. Introduces a new action and associated types in lib
4. Introduces a callsite in ConnectEthereum for the new endpoint

Depends on D7864

Test Plan: I added a console.log of the result on native, and confirmed an already-existing ETH account properly returned the existing userInfo

Reviewers: atul, ginsu

Reviewed By: atul

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D7867