Change plrust.allowed_dependencies to sighup context

[anth0nyleung]

Since there is no technical reason l to set plrust.allowed_dependencies to Postmaster context which requires engine restart to change its value, we can change its context to sighup such that the guc can be modified dynamically to provide more flexibility when using allowlisted crates.

---

Testing

• Origin value is /tmp/old.toml

(23-07-19 16:04:25) [~]
$ > psql -d postgres -c "show plrust.allowed_dependencies"
 plrust.allowed_dependencies
-----------------------------
 /tmp/old.toml
(1 row)

• Change value to /tmp/new.toml

(23-07-19 16:04:27) [~]
$ > vim pg_data/postgresql.conf

(23-07-19 16:04:37) [~]
$ > cat pg_data/postgresql.conf | grep plrust.allowed_dependencies
plrust.allowed_dependencies = '/tmp/new.toml'

• Reload engine and new value is applied

(23-07-19 16:04:43) [~]
$ > psql -d postgres -c "select pg_reload_conf()"
 pg_reload_conf
----------------
 t
(1 row)

(23-07-19 16:04:45) [~]
$ > psql -d postgres -c "show plrust.allowed_dependencies"
 plrust.allowed_dependencies
-----------------------------
 /tmp/new.toml
(1 row)

[eeeebbbbrrrr]

This looks good but does require documentation updates. @anth0nyleung if you don't have time/desire to also do those, that's fine -- we'll take care of it after we merge. I'm really noting it so that it doesn't get forgotten.

[anth0nyleung]

@eeeebbbbrrrr Thanks for review. I have also updated the documentation for the guc. Please feel free to edit further if necessary.