MQTT Trigger enhancement: need topic available for mapping

[opendevpmm]

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[X] feature request
[] support request
[] general question

Current behavior (how does the issue manifest): From the contents of a MQTT message received from outside, only the 'message' field is available for mapping, not the 'topic' field. The 'topic' field also needs to be supported for mapping, since it may have relevant information.

Expected behavior: Both the 'message' and the 'topic' field in the MQTT message should be available for mapping.

Minimal steps to reproduce the problem (not required if feature enhancement): Add a MQTT trigger to the flow and see what is available for mapping to the flow.

What is the motivation / use case for changing the behavior? The MQTT topic can contain useful information that can be used in the flow. Hence it should be available for mapping.

Please tell us about your environment (Operating system, docker version, browser & web ui version, etc): MacOS X version 10.13.5 Docker version 18.03.1-ce, build 9ee9f40 Browser: Firefox Quantum 61.0

Flogo version (CLI & contrib/lib. If unknown, leave empty or state unknown): 0.X.X Flogo version 0.5.3

Additional information you deem important (e.g. issue happens only occasionally): n/a