What this PR does / why we need it:

The Milvus client was always built as MilvusClient(uri=f"{host}:{port}", token=f"{username}:{password}").
API keys couldn't be used, and there was no way to choose a database.

This PR adds three new optional config parameters:

Existing configs build exactly the same client arguments (covered by a test). The docs add a Zilliz
Cloud example that reads the token from an environment variable.

Which issue(s) this PR fixes:

N/A

Checks

• I've made sure the tests are passing.
• My commits are signed off (git commit -s)
• My PR title follows conventional commits format

Testing Strategy

• Unit tests

• Integration tests

• Manual tests

• Testing is not required for this change

• Unit: parametrized client arguments for existing configs, token/uri precedence and db_name;
  uri beats path; path still selects Milvus Lite.

• Server: collections are created in db_name and not in default. Token authentication was
  exercised against Zilliz Cloud with an API key.

Unit tests run on Milvus Lite 3.2.1 (pymilvus 3.0.2). Server tests in sdk/python/tests/integration/online_store/test_milvus_remote.py are marked integration and skip unless ZILLIZ_URI and ZILLIZ_TOKEN are set; they passed against a local Milvus 2.6.0 server and against Zilliz Cloud. The existing Milvus unit and universal integration tests pass unchanged.