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
To complete the pull request process, please assign xuanyang-cn after the PR has been reviewed.
You can assign the PR to them by writing /assign @xuanyang-cn in a comment when ready.

The full list of commands accepted by this bot can be found here.

Details

Needs approval from an approver in each of these files:

• OWNERS

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment

[EeshaaKhan]

/assign @XuanYang-cn