Add LDAP authentication support to Prosody (#50)

This commit adds support for LDAP authentication.

The Dockerfile has been modified to install the required Lua modules (lua-ldap) and the prosody.cfg.lua file has been modified to add environment variables for configuring the LDAP connection. These environment variables include LDAP_BASE, LDAP_SERVER, LDAP_ROOTDN, LDAP_PASSWORD, LDAP_FILTER, LDAP_SCOPE, LDAP_TLS, LDAP_MODE, and LDAP_ADMIN_FILTER.

The authentication variable has also been updated to use the value of the AUTHENTICATION environment variable, which defaults to "internal_hashed" if not set. This allows users to configure Prosody to use LDAP for authentication instead of the default internal hashing method.

* Update test dependencies: aioxmpp, pytest-asyncio + add pytz to requirements.txt

This fixes the following error when running the tests:
E   ModuleNotFoundError: No module named 'pytz'

Instead of pytz only pytz-deprecation-shim was installed.

TODO Check if "pytz" can be removed from requirements.txt later on.

Dockerfile

@@ -35,6 +35,7 @@ RUN apt-get update \
       lua-dbi-mysql \
       lua-expat \
       lua-filesystem \
+      lua-ldap \
       lua-socket \
       lua-sec \
       lua-unbound \