mqtt: Add tls_alpn module option

[agranig]

This is used when a service (like AWS IoT Core) uses one TLS port for multiple services (like https and mqtt), so you have to set the ALPN to 'mqtt' to be able to connect kamailio.

Pre-Submission Checklist

• Commit message has the format required by CONTRIBUTING guide
• Commits are split per component (core, individual modules, libs, utils, ...)
• Each component has a single commit (if not, squash them into one commit)
• No commits to README files for modules (changes must be done to docbook files
  in doc/ subfolder, the README file is autogenerated)

Type Of Change

• Small bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds new functionality)
• Breaking change (fix or feature that would change existing functionality)

Checklist:

• PR should be backported to stable branches
• Tested changes locally
• Related to issue #XXXX (replace XXXX with an open issue number)

Description

Some services use ALPN to have one single TLS port serving multiple protocols, in order to avoid additional round-trips for negotiating TLS connections. https://en.wikipedia.org/wiki/Application-Layer_Protocol_Negotiation has the details.

One of those services using ALPN is AWS IoT Core, which allows components to send messages both via HTTP and MQTT to AWS for further processing (e.g. logging, analysis, storage to db etc). This patch enables kamailio to communicate with this service by allowing the kamailio admin to set the mqtt.so module mod-param tls_alpn to mqtt.