`action_context` field is missing from message type

[nkooman-bzs]

Motivation

Workflow step webhook notifications received an additional context field that should be represented in the models.

Proposed solution

Add action_context field.

Additional context

Should match the docs regarding notification messages.

[IvanKiral]

hey @nkooman-bzs, thanks for the issue, I will look into it and add it.

[IvanKiral]

hey @nkooman-bzs,

whay do you say about this implementation? #35 would this work for you? :)