What this PR does / why we need it:

Builds on #7143, which adds text analysis tags. A String feature can now declare a BM25
index, which Feast creates with the collection:

Field(
    name="title",
    dtype=String,
    tags={
        "milvus.bm25": "true",
        "milvus.bm25_index_params": '{"inverted_index_algo": "TAAT_NAIVE", "bm25_b": 0.3}',
    },
),

• milvus.bm25: "true" adds a SPARSE_FLOAT_VECTOR field <field>_bm25, a BM25 function
  <field>_bm25_fn and a SPARSE_INVERTED_INDEX bm25_index_<field>. Names are derived from the
  field, as for scalar_index_<field> and vector_index_<field>.
• milvus.bm25_index_params passes a JSON object of build parameters to that index.
• Writes leave <field>_bm25 to Milvus: PyMilvus rejects rows that carry a function output.
• A view whose only vector is a BM25 output gets no _placeholder_vector.
• A feature named like a BM25 output is rejected (PyMilvus accepts duplicate field names).
• Retrieval change: retrieve_online_documents_v2 now names the only dense vector field even
  when it is not a requested feature. Before, Feast passed no anns_field and Milvus inferred
  it, which fails with "multiple anns_fields exist" once the collection also has a BM25 output.
  No behaviour change for collections without one.
• Feast does not search the BM25 output yet; clients that query Milvus directly can. A
  follow-up can add BM25 and hybrid search to retrieve_online_documents_v2.
• An existing collection without the output is reported by the existing schema check, and one
  without the index by the missing-index warning.

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: the created output, function, index and build params; no placeholder; validation;
  writes and re-writes through a fresh store on Milvus Lite (fails without the write change);
  the named anns_field (fails without the retrieval change).

• Integration (test_milvus_remote.py, against Milvus 2.5.27, 2.6.27 and 3.0.2): the server stores the
  index params, writes succeed, and a BM25 search on <field>_bm25 finds the row.

• Milvus Lite does not reproduce the anns_field ambiguity, so the unit test checks the argument
  Feast passes.

Misc

Stacked on #7143, whose commit is included here until it merges; review the last commit.
Written with @jonasfagerlund, who is a co-author of the commit.