.NET: Allow injection of Cosmos Client for Chat and State Stores

[Andrew-MS]

Currently from change #1838 Cosmos DB stores initialize their own Cosmos DB client internally using DefaultAzureCredential. Being able to inject our own Cosmos Client allows us to use our own token provider (A security requirement) and custom serializer (used for handling CMK encryption for sensitive fields).

ref: https://github.com/microsoft/agent-framework/tree/main/dotnet/src/Microsoft.Agents.AI.CosmosNoSql

[westey-m]

CC @TheovanKraay