pgcql: sort support

[adamdickmeiss]

https://index-data.atlassian.net/browse/CROSSLINK-228

This is breaking in two ways.

1. The Query interface was modified, so that GetOrderByFields returns string list.
2. A new method Sort was added to the Field interface.

[Copilot]

Pull request overview

Adds CQL sortby support to the pgcql package by generating SQL ORDER BY clauses and exposing ordering metadata on the parsed query object.

Changes:

• Extend Field/Query APIs to support sorting and order-by field introspection.
• Implement parsing and SQL generation for CQL sort specifications (sortby .../sort.*).
• Update unit/integration tests and README usage examples to include ORDER BY generation.

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
pgcql/pgx_test.go	Integration tests now execute queries with ORDER BY appended.
pgcql/pgcql_test.go	Adds test coverage for order-by fields and updates parsing expectations to include ordering.
pgcql/pgcql.go	Public interfaces updated to include sorting support and order-by metadata.
pgcql/pg_query.go	Implements sort spec parsing and ORDER BY SQL clause generation.
pgcql/pg_field_common.go	Provides default sortable expression (column) for fields embedding FieldCommon.
pgcql/pg_combo.go	Explicitly marks combo fields as non-sortable.
README.md	Updates example usage to include ORDER BY in generated SQL.

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