add message size/length to mqtt and mqtt5

[tomkralidis]

Reason/Context

It would be valuable to add message size/length (in bytes) to the MQTT bindings to advertise the maximum message size a given MQTT binding accepts/emits.

Description

Given this is a feature/enhancement, it is not a breaking change. Example definition:

MQTT5:

    bindings:
      mqtt5:
        messageMaxSize: 4096

MQTT:

    bindings:
      mqtt:
        messageMaxSize: 4096

Happy to put forth a PR if the above seems reasonable.

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[pearmaster]

In MQTTv5, there is the "Maximum Packet Size" property on the CONNECT message. See https://docs.oasis-open.org/mqtt/mqtt/v5.0/os/mqtt-v5.0-os.html#_Toc3901050

So I would propose the MQTTv5 bindings follow the same terminology:

bindings:
  mqtt5:
    maximumPacketSize: 4096

I'm hoping to put together a PR for MQTTv5 bindings that includes this and several other MQTTv5 properties (including user properties). I probably will not touch the mqtt (v3) bindings.

[pearmaster]

I've submitted an enhancement ticket for MQTTv5 properties, which would solve the v5 half of this issue: #199

[tomkralidis]

+1. Looking forward to this addition!

[github-actions]

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[tomkralidis]

Update: looks like this is being covered in #201.