Support PostgreSQL CREATE INDEX header variants

[minleejae]

What

• Parse PostgreSQL CREATE INDEX CONCURRENTLY.
• Allow the PostgreSQL index name to be omitted.
• Parse ON ONLY table_name.
• Expose concurrently and only as explicit CreateIndex properties.
• Preserve all variants in both CreateIndex.toString() and CreateIndexDeParser.

Why

These server-valid PostgreSQL header forms currently fail before the index key is reached. They are part of the documented PostgreSQL 18 CREATE INDEX grammar: https://www.postgresql.org/docs/18/sql-createindex.html

Testing

• ./gradlew check --no-daemon --no-build-cache -Dorg.gradle.vfs.watch=false
• Added three parser/deparser and AST tests in PostgreSQLCreateIndexTest.
• Executed the same three statements successfully on PostgreSQL 18.6, including the concurrent build outside a transaction block.

Fixes #2524. The separate key and trailing-clause scope remains in #2525.

AI assistance

OpenAI Codex was used to audit the grammar, draft the implementation, and run validation. The changes and tests were reviewed against the PostgreSQL 18 documentation and a PostgreSQL 18.6 server.