chore: Moving Milvus client to PyMilvus

[franciscojavierarceo]

What this PR does / why we need it:

This PR moves the Milvus Online Store implementation to the PyMilvus Client instead of using Connections, in addition to some other minor changes.

More specifically:

1. sdk/python/feast/feature_store.py:
   • Changed the default value of the distance_metric parameter in the retrieve_online_documents method to "L2".
2. sdk/python/feast/infra/online_stores/milvus_online_store/milvus.py:
   • Replaced the usage of pymilvus.orm.connections with MilvusClient for managing Milvus connections.
   • Added support for username and password configurations in MilvusOnlineStoreConfig.
   • Updated the MilvusOnlineStore class to use MilvusClient for creating and managing collections, including:
     • Creating collections with a composite key.
     • Preparing and creating indexes for vector fields.
     • Handling authentication and connection establishment using username and password.
     • Refactoring methods like _connect, _get_collection, online_write_batch, update, teardown, and retrieve_online_documents to use the new client-based approach.
3. sdk/python/tests/integration/feature_repos/universal/online_store/milvus.py:
   • Replaced MilvusContainer with DockerContainer from testcontainers.core.
   • Added Docker client initialization and log waiting utility for Milvus container readiness.
4. sdk/python/tests/integration/online_store/test_universal_online.py:
   • Re-enabled the previously commented-out integration test for retrieving online documents using Milvus.

Which issue(s) this PR fixes:

Another one in service of #4364

Misc

N/A

[redhatHameed]

/lgtm