[native] Create initial device list during registration

Summary:

• Extracted utility function to generate RawDeviceList from given list of devices and current timestamp.
• Created a function to create and sign initial device list - it creates a singleton array [primaryDeviceID], then signs via Olm.
• Called the function in registerWalletUser and registerPasswordUser functions of identityClient

Depends on D12066

Test Plan:

1. In Identity Service, printed the value of initial_device_list in RPC handlers
2. Registered a password user and a wallet user
3. Confirmed that device list JSON payload is printed in Identity

Reviewers: kamil, will

Reviewed By: kamil

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D12067