alter table set TABLE ACCESS METHOD

I thought this was a good idea, but didn't hear back when I raised it before.
I was motivated to finally look into it by Dilip's toast compression patch,
which also (can do) a table rewrite when changing a column's toast compression.

I called this "set TABLE access method" rather than just "set access method"
for the reasons given on the LIKE thread:
https://www.postgresql.org/message-id/20210119210331.GN8560@telsasoft.com

I've tested this with zedstore, but the lack of 2nd, in-core table AM limits
testing possibilities.  It also limits at least my own ability to reason about
the AM API.  For example, I was surprised to hear that toast is a concept
that's intended to be applied to AMs other than heap.
https://www.postgresql.org/message-id/flat/CA%2BTgmoYTuT4sRtviMLOOO%2B79VnDCpCNyy9rK6UZFb7KEAVt21w%40mail.gmail.com

I plan to add to CF - it seems like a minor addition, but may not be for v14.

https://www.postgresql.org/message-id/20190818193533.GL11185@telsasoft.com
On Sun, Aug 18, 2019 at 02:35:33PM -0500, Justin Pryzby wrote:
>  . What do you think about pg_restore --no-tableam; similar to
>    --no-tablespaces, it would allow restoring a table to a different AM:
>    PGOPTIONS='-c default_table_access_method=zedstore' pg_restore --no-tableam ./pg_dump.dat -d postgres
>    Otherwise, the dump says "SET default_table_access_method=heap", which
>    overrides any value from PGOPTIONS and precludes restoring to new AM.
...
>  . it'd be nice if there was an ALTER TABLE SET ACCESS METHOD, to allow
>    migrating data.  Otherwise I think the alternative is:
>       begin; lock t;
>       CREATE TABLE new_t LIKE (t INCLUDING ALL) USING (zedstore);
>       INSERT INTO new_t SELECT * FROM t;
>       for index; do CREATE INDEX...; done
>       DROP t; RENAME new_t (and all its indices). attach/inherit, etc.
>       commit;
>
>  . Speaking of which, I think LIKE needs a new option for ACCESS METHOD, which
>    is otherwise lost.