[lib] scaffolding for delete identity account

Summary: introduce some scaffolding for deleteIdentityAccount

Test Plan:
tested the following scenarios:

account deletion on native with valid CSAT -> account deleted on keyserver and identity service
account deletion on native with invalid CSAT -> account deleted on keyserver and saw DELETE_IDENTITY_ACCOUNT_FAILED in devtools
account deletion on native while disconnected from identity service -> account deleted on keyserver and saw DELETE_IDENTITY_ACCOUNT_FAILED in devtools

Reviewers: ashoat, inka, tomek

Reviewed By: ashoat, inka, tomek

Subscribers: ashoat, inka, tomek

Differential Revision: https://phab.comm.dev/D10202