Merge statement

### Is your feature request related to a problem or challenge?

Postgres has merge statement

> MERGE — conditionally insert, update, or delete rows of a table

https://www.postgresql.org/docs/current/sql-merge.html

Duckdb has the open request https://github.com/duckdb/duckdb/discussions/4601

I think it would be nice if we have one

### Describe the solution you'd like

Support MERGE command

### Describe alternatives you've considered

_No response_

### Additional context

_No response_

Start by reviewing the PostgreSQL MERGE documentation and the linked DuckDB discussion to understand the requested command's scope. Trace DataFusion's SQL command entry points and determine the supported conditional insert, update, and delete behavior. Done means MERGE is supported with appropriate coverage for those operations.

Written by the indexing model from the issue text.