Add CockroachDB PeerType support

[viragtripathi]

Added CockroachDB as a supported peer type following the same pattern as other database types (ClickHouse, Elasticsearch, etc.). This enables PeerDB integration with CockroachDB.

Changes

• Added COCKROACHDB keyword to src/keywords.rs
• Added CockroachDB variant to PeerType enum in src/ast/mod.rs
• Updated Display impl for PeerType
• Updated parse_peer_type function in src/parser/mod.rs

Testing

• All existing tests pass
• Code formatted with cargo fmt