[identity] Introduce PlatformDetails DDB attribute

Summary:
Resolves this comment
We want to store Redux state version and Desktop version (full type PlatformDetails) in DDB.

Test Plan:

• Registered a device before this diff
• Registered a device with this diff applied

Then:

• Used awslocal dynamodb scan --table-name identity-devices to make sure platform details are there
• Modified GetInboundKeys RPC handler to print out devices data to console when fetched them from DDB. Made sure PlatformDetails is present and correct for both devices.

Reviewers: marcin, varun, will

Reviewed By: marcin

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D12269