Add support for asynchronous fetching, the `async_capable` table option

Add support for the `async_capable` option to allow async fetching of result sets, as described in [All Your GUCs in a Row: `enable_async_append`](https://thebuild.com/blog/all-your-gucs-in-a-row-enableasyncappend/) and the [FDW Routines for Asynchronous Execution](https://www.postgresql.org/docs/18/fdw-callbacks.html#FDW-CALLBACKS-ASYNC) docs. The option was added to postgres_fdw in postgres/postgres@27e1f1456 and modified in postgres/postgres@bb684c82 and postgres/postgres@5c854e7.

This will allow Postgres to execute fetches via pg_clickhouse asynchronously with other foreign tables, via pg_clickhouse or otherwise. Note, however, that its utility will be limited for fetches from the same foreign server, as pg_clickhouse uses a a single connection for all queries for a given server, so they'll run synchronously. Thus this feature, while desirable, is less important than improving join pushdown in general.

Suggested by @heavycrystal.

Start with PostgreSQL's postgres_fdw changes referenced in commits 27e1f1456, bb684c82, and 5c854e7, then read the FDW Routines for Asynchronous Execution documentation. Trace pg_clickhouse's foreign-table option handling and fetch entry points; done means async_capable is supported and fetches can execute asynchronously alongside other foreign tables.

Written by the indexing model from the issue text.