Feat: Add label filter support in pgdiskann client

[EeshaaKhan]

This PR adds support for label-based filtering in pgdiskann client.

Changes introduced:

• Added support for FilterOp.StrEqual to enable filtering by scalar string labels.
• Introduced optional with_scalar_labels flag to create tables with an additional label column.
• Updated table creation logic to conditionally include scalar label support.
• Extended insert_embeddings to handle label data when enabled.
• Implemented prepare_filter() to dynamically construct the WHERE clause based on filter type.
• Unified search query generation through _generate_search_query() to support filtered and unfiltered search paths.
• Existing numeric filtering (id >= value) and non-filtered search behavior remain unchanged.

[sre-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: EeshaaKhan
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[EeshaaKhan]

/assign @XuanYang-cn

[EeshaaKhan]

@XuanYang-cn Could you please review this PR?

[EeshaaKhan]

@alwayslove2013 Hi, this PR has been pending review for a while. Could you please take a look?

[XuanYang-cn]

Hi @EeshaaKhan, thanks for the contribution, and apologies for the delayed response! I'll review this PR shortly.

Before I do, could you please rebase onto the latest main? We've since upgraded to Pydantic v2 and updated several checks, so this PR didn't trigger the GitHub Actions workflow and may fail against the current codebase.
Thanks again for your patience!