CASSANDRA-20943 Introducing comments and security labels for schema elements

[jyothsnakonisa]

This PR adds support for Comments and Security Labels on schema elements, as proposed in
CEP-52: Schema Annotations for Apache Cassandra.

What’s Included

• Implementation for storing comments and security labels on keyspaces, tables, columns, and UDTs.
• Updates to the CQL grammar to introduce new DDL statements for defining and managing schema annotations.
• Two new virtual tables — one for comments and another for security labels — to make it easy to query annotations.
• Autocomplete improvements for annotation-related commands, including support within USE KEYSPACE context.
• Comprehensive tests covering storage, queries, and grammar changes to ensure correctness and stability.