What this PR does / why we need it:

#6940 added partition keys. Milvus has two per-collection settings that go with them and that can
only be set when the collection is created: the number of partitions, and
partition key isolation,
which makes a search filtered on the key scan only that key's data. Like key_ttl_seconds in the
Redis store, they are settings the store applies when it creates data and cannot change later.

Both are ignored for feature views without a partition key, and both only apply when Feast creates
the collection, like partition_key itself. When unset nothing is sent, so Milvus keeps its
defaults. Milvus Lite ignores num_partitions.

online_store:
  type: milvus
  partition_key: "brand_id"
  num_partitions: 32
  partition_key_isolation: true
  index_type: "HNSW"

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 (mocked client): both settings are sent for a feature view with a partition key and not
  for one without; nothing is sent when unset; num_partitions: 0 is rejected.

• Server: the collection reports the partition count and the isolation property, and a filtered
  search returns only that tenant's rows.

Unit tests run on Milvus Lite 3.2.1 (pymilvus 3.0.2). The server test in
sdk/python/tests/integration/online_store/test_milvus_remote.py passes against a local
Milvus 3.0.2 server. The existing Milvus unit and server tests pass unchanged.

Misc

Part of a stack that adds to the Milvus online store the update and delete functionality the Redis
online store has, following #6882, #6895, #6917, #6921 and #6940. The other PRs of the stack are
#7082 (row deletion) and #7084 (tie-break in write batches). mutation_timeout and a schema mismatch check follow once these merge.