Additional Internal Event properties documentation and generator

Problem

After implementing the possibility to pass additional properties to Internal Events in Additional Internal event properties on Frontend (#434505 - closed), Additional internal event properties on Backend (#434504 - closed) and Update additional properties handling in event ... (#434563 - closed) and filtering for those Allow to filter by additional properties in Ser... (#435338 - closed) we need to also update our documentation and the interactive generator to make our users aware of the new possibilities.

Desired Outcome

Interactive generator and documentation are updated to expose the new possibilities to add additional properties and filter metrics on those.

Proposed solution

Add interactions to 'Context' screen of the CLI.

After collecting identifiers, print:

Events triggered on gitlab.com can be viewed/sorted/grouped live in Snowplow. Optionally, we can save extra info from each event to a few of the unreserved fields. 2 additional strings and 1 numeric value are available for each event. 

  ex) Event: change_merge_request_status
      String: opened || merged || closed
      Description: Status of merge request after update      

Until user selects 'Not applicable' or all 3 extra slots are filled, repeat these prompts:

Select one: If applicable, which type of field do you want to add to the event? 
> N/A
> String
> Number

Input text: Describe what the field will include: (enter to skip)

Disable or remove options from the list as they become unavailable.

• Update interactive generator to ask for additional properties
• Update documentation

Follow up

• Announce the new capabilities of internal events in #engineering-fyi