feat: sink external user id

[Ian2012]

Description: This PR implements a sink for the ExternalID model

Installation instructions: After configuring Aspects, you should run the following SQL in CLickhouse:

CREATE TABLE event_sink.external_id
(
    `external_user_id` UUID NOT NULL,
    `external_id_type` String,
    `user_id` Int32,
    `dump_id` UUID NOT NULL,
    `time_last_dumped` String NOT NULL
)
ENGINE = MergeTree
PRIMARY KEY (external_user_id, time_last_dumped)
ORDER BY (external_user_id, time_last_dumped)

Testing instructions:

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

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

[openedx-webhooks]

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[bmtcril]

This will be good when you get that last bit of coverage. I created a new ticket in this repo for adding migrations.

[openedx-webhooks]

@Ian2012 🎉 Your pull request was merged! Please take a moment to answer a two question survey so we can improve your experience in the future.