Including a sample Table Access Method with core code

At PgCon 2023 in Ottawa we had an Unconference session on Table Access
Methods [1]

One thing that was briefly mentioned (but is missing from the notes)
is need to have a sample API client in contrib/ , both for having a
2nd user for API to make it more likely that non-heap AMs are doable
and also to serve as an easy starting point for someone interested in
developing a new AM.

There are a few candidates which could be lightweight enough for this

* in-memory temp tables, especially if you specify max table size at
creation and/or limit data types which can be used.

* "overlay tables" - tables which "overlay" another - possibly
read-only - table and store only changed rows and tombstones for
deletions. (this likely would make more sense as a FDW itself as Table
AM currently knows nothing about Primary Keys and these are likely
needed for overlays)

* Table AM as a (pl/)Python Class - this is inspired by the amazing
Multicorn [2] FDW-in-Python tool which made it ridiculously easy to
expose anything (mailbox, twitter feed, git commit history,
you-name-it) as a Foreign Table

Creating any of these seems to be a project of size suitable for a
student course project or maybe Google Summer of Code [3].

Included Mark Dilger directly to this mail as he mentioned he has a
Perl script that makes a functional copy of heap AM that can be
compiled as installed as custom AM.

@mark - maybe you can create 3 boilerplate Table AMs for the above
named `mem_am`, `overlay_am` and `py3_am` and we could put them
somewhere for interested parties to play with ?

[1] https://wiki.postgresql.org/wiki/PgCon_2023_Developer_Unconference#Table_AMs
[2] https://multicorn.org/ - unfortunately unmaintained since 2016,
but there are some forks supporting later PostgreSQL versions
[3] https://wiki.postgresql.org/wiki/GSoC - Google Summer of Code

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Best Regards
Hannu