API to customise postgres.conf

[samwillis]

We need a way to let users set a custom postgres.conf.

Some extensions also need to have specific things set in postgres.conf and it would be good if that could be done automatically when enabling them.

[pmp-p]

That file only appears - after - initdb has run, so maybe just concat the extra conf file to it right before restart. Question is that config a user one ( that could be changed in pglite options) or specific defaults to extension and then could be in the tarball ( and applied before the dynamic link because at that stage no PGDATA/*conf has been read yet).

[pmp-p]

i guess we should support both but some element of configuration are tricky because they could concern multiple extensions for the same appended line. eg https://www.postgresql.org/docs/current/runtime-config-client.html#RUNTIME-CONFIG-CLIENT-PRELOAD related to #189