MilvusVectorStore: Add support for nested MetadataFilters and FilterOperator.IS_EMPTY

[RasmusAntons]

Description

Allows nested MetadataFilters by recursively evaluating them and wrapping the result in parentheses.

For example:

filters = MetadataFilters(
    filters=[
        MetadataFilter(key="a", value=1, operator=FilterOperator.EQ),
        MetadataFilters(
            filters=[
                MetadataFilter(key="b", value=2, operator=FilterOperator.EQ),
                MetadataFilter(key="c", value=3, operator=FilterOperator.EQ)
            ],
            condition=FilterCondition.OR
        )
    ],
    condition=FilterCondition.AND
)
expr = _to_milvus_filter(filters)
assert expr == "(a == 1 and (b == 2 or c == 3))"

Also allows using FilterOperator.IS_EMPTY, which is translated to array_length(key) == 0.

Fixes #16310

Version Bump?

Did I bump the version in the pyproject.toml file of the package I am updating? (Except for the llama-index-core package)

• Yes
• No

Type of Change

Please delete options that are not relevant.

• New feature (non-breaking change which adds functionality)

How Has This Been Tested?

Please describe the tests that you ran to verify your changes. Provide instructions so we can reproduce. Please also list any relevant details for your test configuration

• Added new unit/integration tests
• Added new notebook (that tests end-to-end)
• I stared at the code and made sure it makes sense

Suggested Checklist:

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added Google Colab support for the newly added notebooks.
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• I ran make format; make lint to appease the lint gods

[RasmusAntons]

Looking at the comment for FilterOperator.IS_EMPTY, the filter should match if "the field is not exist or empty (null or empty array)". I'm only checking for an empty array, I'll see if I can extend that to nonexistent/null fields.