[keyserver] processOlmAccountCreation

Summary:
similar to SIWE account creation logic. differences:

• signedIdentityKeysBlob must be present
• we use the UserIdentifier type to get the username (and eth address if present)
• we don't send a reserved username request since this user has already registered with identity

not deduping because we eventually want to deprecate the siwe account creation function anyway

Depends on D10491

Test Plan: tested in next diff by calling the olm auth responder and confirming that the above listed changes all work as expected

Reviewers: ashoat, atul

Reviewed By: ashoat

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D10492