Add Slack archive plus instructions and required dependency for viewing

[merobi-hub]

Problem

Being on Slack's free plan means that we lack access to threads more than 3 months old.

Solution

This adds a small Slack archive and open source viewer to dev for testing/experimentation. A different approach might be preferable, but having the archive and dependencies in dev makes it fairly easy to test a variety of solutions. Note: a small patch might be necessary in order to use this first tool (slack-export-viewer). There are details in the repo here: hfaran/slack-export-viewer#165. If we like this approach, an archive of the entire history can be added in the future.

Note: All schema changes require discussion. Please link the issue for context.

• Your change modifies the core OpenLineage model
• Your change modifies one or more OpenLineage facets

If you're contributing a new integration, please specify the scope of the integration and how/where it has been tested (e.g., Apache Spark integration supports S3 and GCS filesystem operations, tested with AWS EMR).

One-line summary: adds slack archive and viewer to dev

Checklist

• You've signed-off your work
• Your pull request title follows our guidelines
• Your changes are accompanied by tests (if relevant)
• Your change contains a small diff and is self-contained
• You've updated any relevant documentation (if relevant)
• Your comment includes a one-liner for the changelog about the specific purpose of the change (if necessary)
• You've versioned the core OpenLineage model or facets according to SchemaVer (if relevant)
• You've added a header to source files (if relevant)

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SPDX-License-Identifier: Apache-2.0
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