[identity] Add RPC for secondary device login

Summary:
This diff adds a RPC that is called by secondary device, after receiving information from primary device that it has been added to the device list.

This RPC serves two purposes:

• Perform a device key upload for newly added secondary device
• Mint an access token for the secondary device

The implementation is done in the next diff

Depends on D10800

Test Plan:

• Identity service and grpc_cliends compile
• Flow for grpc-web codegen

Reviewers: ashoat, varun, michal

Reviewed By: varun

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D10861