Add support for PostgreSQL backend

[jmole]

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• I have read the contributing guideline and understand that I have made the correct modifications

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Description:

This PR adds support for the PostgreSQL backend to the minetest server build.

Benefits of this PR and context:

PostgreSQL is a more performant database backend for minetest servers. The default database backend is sqlite3, which can bottleneck your server under high client load.

To use the PostgreSQL backend, you must modify world.mt. This will be created in the worlds/world directory by default when the container is started.

A sample configuration might be as follows:

# Commented out defaults
# mod_storage_backend = sqlite3
# auth_backend = sqlite3
# player_backend = sqlite3
# backend = sqlite3
mod_storage_backend = postgresql
auth_backend = postgresql
player_backend = postgresql
backend = postgresql
pgsql_connection =             host=your-db-hostname port=5432 user=postgres password=hunter2 dbname=minetest
pgsql_player_connection =      host=your-db-hostname port=5432 user=postgres password=hunter2 dbname=minetest
pgsql_auth_connection =        host=your-db-hostname port=5432 user=postgres password=hunter2 dbname=minetest
pgsql_mod_storage_connection = host=your-db-hostname port=5432 user=postgres password=hunter2 dbname=minetest

In this example, you would be running a PostgreSQL server at "your-db-hostname", with admin user/password postgres/hunter2. The database "minetest" doesn't have to exist, provided that the user "postgres" has sufficient rights to create a database.

How Has This Been Tested?

Image has been tested on x86_64 Ubuntu 22.04, and PostgreSQL support has been verified both in the build logs, and in the final server binary.

Source / References:

https://wiki.minetest.net/Database_backends#PostgreSQL

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[LinuxServer-CI]

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https://ci-tests.linuxserver.io/lspipepr/minetest/5.8.0-pkg-3c317f08-dev-3185dfdf749ee229dc9a7e0b4e1eb9e228fa511f-pr-55/index.html
https://ci-tests.linuxserver.io/lspipepr/minetest/5.8.0-pkg-3c317f08-dev-3185dfdf749ee229dc9a7e0b4e1eb9e228fa511f-pr-55/shellcheck-result.xml

Tag	Passed
amd64-5.8.0-pkg-3c317f08-dev-3185dfdf749ee229dc9a7e0b4e1eb9e228fa511f-pr-55	✅
arm64v8-5.8.0-pkg-3c317f08-dev-3185dfdf749ee229dc9a7e0b4e1eb9e228fa511f-pr-55	✅

[LinuxServer-CI]

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