Split json_object module

Currently the json_object module contains all structs that are used for serialisation/deserialisation.

They can be categorised as follows:

• Identity key pair of an olm_rs::account::OlmAccount
• Device keys for uploading to the Matrix server, as described here
• One-time-keys for uploading to the Matrix server, as described here

The amount of JSON objects that will have to be used together with serde will only increase as the project continues, so splitting them seems like a good idea to me.