Implement json.Marshaler and Unmarshler for data.Type

[balamg]

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[x] enhancement request
[] feature request
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Current behavior (how does the issue manifest):

Expected behavior:

Minimal steps to reproduce the problem (not required if feature enhancement):

What is the motivation / use case for changing the behavior?
Without this, have to write a custom marshaler for my wrapping struct, just to convert from the enumeration's string representation to its enum value

Please tell us about your environment (Operating system, docker version, browser & verison if webui, etc):

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

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