Added support for mongoDB KV store

[shrinitg]

Added the support for mongoDB as KV store
validated in mongodb, it is able to store agent data, session data and turn data

this is how run.yaml would look:

    config:
      persistence_store:
        type: mongodb
        namespace: null
        host: localhost
        port: 27017
        db: llamastack
        user: ""
        password: ""
        collection_name: llamastack_kvstore

[ashwinb]

you should also add a **kwargs parameter to sample run config (since the distro_codegen.py script invokes it with a "distro" parameter)

[ashwinb]

This looks cool. I have a minor comment inline. Are you able to add some tests? I believe we have some KVStore related tests...