[lib] Introduce SignedIdentityKeysBlob[ServerRequest/ClientResponse] types

Summary:
Add types to lib/types/request-types.js. Pretty straightforward, matched what was done for MoreOneTimeKeys.

Found the ClientServer/ServerServer/ServerClient/ClientClient naming convention to be a bit weird... but fine for now.

Test Plan: NA/flow/will be tested with subsequent diffs.

Reviewers: ashoat, tomek

Reviewed By: ashoat

Differential Revision: https://phab.comm.dev/D6951