Cleaner activity and trigger interface for Go API

[mellistibco]

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Current behavior: Currently, the activity/trigger interface available via the Go API exposes input & output objects as a map, which makes it rather difficult to leverage IDE tooling such as autocomplete, etc and it also means that you'll need to cross reference the activity.json for the metadata names, etc.

Expected behavior: Expose a strongly typed interface to simplify the usage of activities and triggers via the Go API