nixos/postgresql: turn `settings` into a submodule

The main idea behind that was to be able to do more sophisticated
merging for stuff that goes into `postgresql.conf`:
`shared_preload_libraries` is a comma-separated list in a `types.str`
and thus not mergeable. With this change, the option accepts both a
comma-separated string xor a list of strings.

This can be implemented rather quick using `coercedTo` +
freeform modules. The interface still behaves equally, but it allows to
merge declarations for this option together.

One side-effect was that I had to change the `attrsOf (oneOf ...)` part into
a submodule to allow declaring options for certain things. While at it,
I decided to move `log_line_prefix` and `port` into this structure as
well.

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