feat(realtime): add realtime filter builder

[jan-tennert]

What kind of change does this PR introduce?

Feature (closes #1343)

This pull request adds a new fluent builder API for constructing Postgres changes filter strings in Realtime channels, aligning filter support with PostgREST and improving developer experience. It introduces the RealtimePostgresFilterBuilder class and related types, expands filter operator support, and updates compliance documentation. Comprehensive tests are included to ensure correct behavior.

Realtime Postgres filter builder API:

• Added the RealtimePostgresFilterBuilder class, which provides a fluent DSL for building filter strings for Postgres changes subscriptions in Realtime, mirroring the postgrest-kt filter API. This includes methods for all supported operators, automatic value escaping, and support for combining multiple conditions.
• Integrated the builder into PostgresChangeFilter, allowing users to construct filters using the new DSL.

Filter operator support and mapping:

• Introduced the RealtimePostgresChangesFilterOperator enum to enumerate all supported filter operators for Realtime Postgres changes, and provided a mapping to FilterOperator.

• Added support for the ISDISTINCT operator in both FilterOperator and the filter value escaping logic.
  Testing and compliance:

• Added comprehensive tests for the new builder and operators in RealtimePostgresFilterBuilderTest and extended tests for ISDISTINCT in PostgrestFilterBuilderTest.

• Updated sdk-compliance.yaml to mark realtime.subscriptions.postgres_changes_filter as fully implemented and list all new API symbols.