core.configmanager: Add ways to read config values from files

Inspired by something MattJ said

Allows retrieving config values from files which are expected to be
relative to the config directory, extending on the ENV_ method of
retrieving config values from outside the config file.

- FileLine retrieves the first line, stripping any trailing newline
- FileContents reads the whole file
- FileLines reads lines into an array

spec/scansion/admins.txt

@@ -0,0 +1 @@
+admin@localhost

spec/scansion/prosody.cfg.lua

@@ -1,6 +1,6 @@
 --luacheck: ignore
 
-admins = { "admin@localhost" }
+admins = FileLines("admins.txt")
 
 network_backend = ENV_PROSODY_NETWORK_BACKEND or "epoll"
 network_settings = Lua.require"prosody.util.json".decode(ENV_PROSODY_NETWORK_SETTINGS or "{}")