Add annotation syntax to pg_hba.conf entries

Hi,

I'm opening this thread after a brief discussion regarding a potential 
new syntax to enable annotations in pg_hba entries. [1]

This feature mainly aims to annotate pg_hba entries in a way that the 
annotations can be parsed and displayed in the pg_hba_file_rule view for 
reporting purposes. For instance, these annotations could contain 
information like tags, client (application) names or any relevant info 
regarding the granted access.

Initially I explored the possibility of using the inline comments after 
a '#', but there were a few valid concerns to this approach [2]

hostssl  db  jim  127.0.0.1/32  cert  map=foo  # comment

I had previously thought of introducing a new character do identify such 
annotations, e.g [] ... but the necessary changes in the hba.c to add 
this feature could add too much complexity to the code. [3]

Perhaps a "less controversial" option would be to add a new variable, 
just like with user name maps.

hostssl  db  jim  127.0.0.1/32  cert  map=foo  annotation=comment
hostssl  db  jim  127.0.0.1/32  cert  map=bar annotation="comment"

Any thoughts?

Thanks!

Jim

1- 
https://www.postgresql.org/message-id/flat/4d623899-36ac-71b5-311d-2a4672d75736@uni-muenster.de
2- 
https://www.postgresql.org/message-id/E543222B-DE8D-4116-BA67-3C2D3FA83110%40yesql.se
3- 
https://www.postgresql.org/message-id/flat/ZPHAiNp%2ByKMsa/vc%40paquier.xyz#05a8405be272342037538ee432d92884