Re: Transparent column encryption

To kick some things off for PG18, here is an updated version of the 
patch for automatic client-side column-level encryption.  (See commit 
message included in the patch for a detailed description, if you have 
forgotten.  Also, see [0] if the thread has dropped off your local mail 
storage.)

[0]: 
https://www.postgresql.org/message-id/flat/89157929-c2b6-817b-6025-8e4b2d89d88f@enterprisedb.com

This patch got stuck around CF 2023-03 because expanding the size of the 
tuple descriptor (with new pg_attribute columns) had a noticeable 
performance impact.  Various work in PG17 has made it more manageable to 
have columns in pg_attribute that are not in the tuple descriptor, and 
this patch now takes advantage of that (and I wanted to do this merge 
soon to verify that the changes in PG17 are usable).  Otherwise, this 
version v20 is functionally unchanged over the last posted version v19. 
Obviously, it's early days, so there will be plenty of time to have 
discussions on various other aspects of this patch.  I'm keeping a keen 
eye on the discussion of protocol extensions, for example.