mqtt should be implemented as an API, not hardcoded into vzlogger core

[r00t-]

i had not paid much attention to the mqtt implementation before,
but in #538 it came up that aggregation does not work for mqtt.

this is because mqtt (in #357) was not implemented as an api,
but hardcoded directly into the vzlogger core, bypassing aggregation and possibly other logic: https://github.com/mbehr1/vzlogger/blob/4ddc311c19ee7558c54049b2097ef5316783864a/src/threads.cpp#L143

mqtt should be an api so it can be configured like others, and make use of other features of vzlogger.
it should should look like this (at vzlogger.conf level):

    "meters": [{
        "channels": [{
            "api": "mqtt",

[r00t-]

i might make a draft of how to change this,
but i'm not a user of or experienced in mqtt, so i won't be able to complete this by myself.