feat: add Medatada as data attribute

[mariajgrimaldi]

Description:

This PR modifies the metadata object sent when triggering an Open edX event.

JIRA: Link to JIRA ticket

Dependencies:
This PR depends on #8

Merge deadline: List merge deadline (if any)

Installation instructions: List any non-trivial installation
instructions.

Testing instructions:

1. Install openedx-events using the current branch
2. Go to Django shell:

>>> from openedx_events.tooling import OpenEdxPublicSignal
>>> public_signal = OpenEdxPublicSignal(event_type="org.openedx.learning.session.login.completed.v1", data={'user': None})
>>> public_signal.generate_signal_metadata()
{'id': UUID('b6af26cc-ee39-11eb-8988-0242ac19000b'), 'event_type': 'org.openedx.learning.session.login.completed.v1', 'minorversion': 0, 'time': datetime.datetime(2021, 7, 26, 17, 48, 41, 41263), 'source': 'openedx/lms/web', 'sourcehost': 'lms.devstack.edx', 'sourcelib': (0, 1, 3)}

Reviewers:

• tag reviewer
• tag reviewer

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.
• Delete working branch (if not needed anymore)

Author concerns:

[felipemontoya]

This PR was only a POC right? Once the EventsMetadata attr class was added to #7, this is no longer needed. Can we close it?