Extensible storage manager API - smgr hooks

Hi, hackers!

Many recently discussed features can make use of an extensible storage
manager API. Namely, storage level compression and encryption [1], [2], [3],
disk quota feature [4], SLRU storage changes [5], and any other features
that may want to substitute PostgreSQL storage layer with their
implementation (i.e. lazy_restore [6]).

Attached is a proposal to change smgr API to make it extensible. The idea
is to add a hook for plugins to get control in smgr and define custom
storage managers. The patch replaces smgrsw[] array and smgr_sw selector
with smgr() function that loads f_smgr implementation.

As before it has only one implementation - smgr_md, which is wrapped into
smgr_standard().

To create custom implementation, a developer needs to implement smgr API
functions
static const struct f_smgr smgr_custom =
{
.smgr_init = custominit,
...
}

create a hook function
const f_smgr * smgr_custom(BackendId backend, RelFileNode rnode)
{
//Here we can also add some logic and chose which smgr to use based
on rnode and backend
return &smgr_custom;
}

and finally set the hook:
smgr_hook = smgr_custom;

[1]
https://www.postgresql.org/message-id/flat/11996861554042351(at)iva4-dd95b404a60b(dot)qloud-c(dot)yandex(dot)net
[2]
https://www.postgresql.org/message-id/flat/272dd2d9.e52a.17235f2c050.Coremail.chjischj%40163.com
[3] https://postgrespro.com/docs/enterprise/9.6/cfs
[4]
https://www.postgresql.org/message-id/flat/CAB0yre%3DRP_ho6Bq4cV23ELKxRcfhV2Yqrb1zHp0RfUPEWCnBRw%40mail.gmail.com
[5]
https://www.postgresql.org/message-id/flat/20180814213500.GA74618%4060f81dc409fc.ant.amazon.com
[6] https://wiki.postgresql.org/wiki/PGCon_2021_Fun_With_WAL#Lazy_Restore

--
Best regards,
Lubennikova Anastasia