[AIRFLOW-4162] Support layout blocks in Slack operators

[bskim45]

Jira

• My PR addresses the following Airflow Jira issues and references them in the PR title.
  • https://issues.apache.org/jira/browse/AIRFLOW-4162

Description

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This PR adds support for Slack layout blocks in Slack operators: SlackAPIPostOperator, SlackWebhookOperator
https://api.slack.com/reference/block-kit/blocks

This PR also compatible with slack python sdk v1 which we are using, since slack SDK simply passes kwargs dict to the api client as a payload.

Tests

• My PR adds the following unit tests OR does not need testing for this extremely good reason:

Updated corresponding operator tests

Commits

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  1. Subject is separated from body by a blank line
  2. Subject is limited to 50 characters (not including Jira issue reference)
  3. Subject does not end with a period
  4. Subject uses the imperative mood ("add", not "adding")
  5. Body wraps at 72 characters
  6. Body explains "what" and "why", not "how"

Documentation

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  • All the public functions and the classes in the PR contain docstrings that explain what it does
  • If you implement backwards incompatible changes, please leave a note in the Updating.md so we can assign it to a appropriate release

[feluelle]

Hey @bskim45, I am sorry, but would it be okay for you if we close this PR and add you as an co-author to #6418 ? It does basically the same and we already started to review the other one. :/

cc @mingrammer

[bskim45]

No problem @feluelle. I will close this PR.

[feluelle]

I know, your PR was opened first.. :/

[mingrammer]

@bskim45 I'm sorry.. I didn't check there is already a PR for this issue.