[identity] db client method to get session initialization info

Summary:
the session initialization info will be stored in a map in dynamoDB. the map's key is the device signingPublicKey, and the value is a map of initialization info component names (e.g. social proof, one-time prekeys, etc.) to their respective values.

example json (omitting irrelevant fields):

{
"userID": "12345",
"devices": {

"signingkey1": {
 "otherkey1": "lorem",
 "socialproof": "ipsum"
}

}
}

Test Plan:
created 3 items in a test DDB table:

• well-formed item with a devices map that contained a couple signing keys and their corresponding initialization info components
  • got back the expected HashMap<String, HashMap<String, String>>
• well-formed item with an empty devices map
  • got back the expected HashMap, but with nothing inside
• malformed item with no devices attribute
  • got back an error, as expected

Reviewers: bartek, jon, tomek

Reviewed By: bartek, jon

Subscribers: ashoat, atul

Differential Revision: https://phab.comm.dev/D6899