Thread flags through begin-scan APIs

Thread flags through begin-scan APIs

Add an AM user-settable flags parameter to several of the table scan
functions, one table AM callback, and index_beginscan(). This allows
users to pass additional context to be used when building the scan
descriptors.

For index scans, a new flags field is added to IndexFetchTableData, and
the heap AM saves the caller-provided flags there.

This introduces an extension point for follow-up work to pass per-scan
information (such as whether the relation is read-only for the current
query) from the executor to the AM layer.

Author: Melanie Plageman <melanieplageman@gmail.com>
Reviewed-by: Andres Freund <andres@anarazel.de>
Reviewed-by: Tomas Vondra <tomas@vondra.me>
Reviewed-by: David Rowley <dgrowleyml@gmail.com>
Reviewed-by: Chao Li <li.evan.chao@gmail.com>
Discussion: https://postgr.es/m/2be31f17-5405-4de9-8d73-90ebc322f7d8%40vondra.me