.NET: Consider making ChatMessageStore and AIContextProvider Factories plus GetNewThread and DesriailzeThread async

[westey-m]

User Scenario:

The developer already has a database full of chat history entries for different threads.
The developer also has a custom ChatMessageStore implementation that can read from this database.
The developer has not used Agent Framework before, so when creating a thread, needs to lookup the appropriate chat history for each user in the database, potentially using the user's name as a secondary index.

Once the developer has bootstrapped an AgentThread, they are happy to store the id from the database with the agent thread state, but to bootstrap it they need to do a lookup.
They want to do this lookup in the ChatMessageStore factory.

Implications:

Changing the factory methods to be async, will also require us to change GetNewThread and DeserializeThread to be async, since both of these call the factory.