feat: add xapi transformer for completion events

[Ian2012]

Description: Adds an xAPI transformer for completion events

JIRA: Link to JIRA ticket

Dependencies: openedx/completion#245

Merge deadline: List merge deadline (if any)

Installation instructions: List any non-trivial installation
instructions.

Testing instructions:

1. Open page A
2. Do thing B
3. Expect C to happen
4. If D happened instead - check failed.

Merge checklist:

• All reviewers approved
• CI build is green
• Version bumped
• Changelog record added
• Documentation updated (not only docstrings)
• Commits are squashed

Post merge:

• Create a tag
• Check new version is pushed to PyPI after tag-triggered build is
  finished.

Author concerns: List any concerns about this PR - inelegant
solutions, hacks, quick-and-dirty implementations, concerns about
migrations, etc.

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[ziafazal]

@Ian2012 overall looks good. Couple of things to take care of

1. One things we need to do is change schema of xAPI statement and put progress under result instead of extensions of object.
2. Documentation needs to added here and here

[ziafazal]

definition of object should be like below and progress should be under result

"object": {
 "id": "http://localhost:18000/xblock/block-v1:edX+DemoX+Demo_Course+type@problem+block@7c54b16c8ed34f9f8772015178c7a175",
"objectType": "Activity",
"definition": {
      "type": "http://id.tincanapi.com/activitytype/resource"
    }
  },
  "result": {
    "extensions": {
      "https://w3id.org/xapi/cmi5/result/extensions/progress": 100.0
    }
  },

[Ian2012]

thanks, I've updated the statement cc @bmtcril

[ziafazal]

@Ian2012 could you please update the documentation files to include new events?

[ziafazal]

👍

[bmtcril]

@Ian2012 should be good to go if you squash and bump the version

[Ian2012]

I will, remember that this event is not approved yet in openedx/completion#245

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