Make user agent configurable post client creation

[amanrao23]

Currently there is no way to change user agent once CosmosClient has been created.
This was a not a requirement earlier as user-agents need not be reconfigured.

But with addition of CosmosDB Integrations with AI libraries like Langchain and LlamaIndex, we want to track the adoption.

Users can pass the CosmosClient instance itself during initialization of different stores (eg. vectorStore) which leads to default value of user-agent getting used and does not help in tracking adoption of these AI libraries.

Also, if connectionString is passed, there is no way to set other CosmosClientOptions.

Ask is to support both these changes.