[AIRFLOW-5749] Support the "blocks" component for the Slack operators

[mingrammer]

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Slack clients v1.x.x also does support "blocks".

[ashb]

Change looks good, but test failrues are I think fixed on master. Can you rebase on to latest master?

[mingrammer]

@ashb I'm done!

[mingrammer]

I found a lint error and fixed it

[feluelle]

Oh we have two of those #6402

[mingrammer]

Oh, I didn't see that :(

[feluelle]

LGTM, but please add [AIRFLOW-4162] to the commit title and a Co-authored-by: to the commit message, because it refers to the same feature. Thanks.

See https://help.github.com/en/github/committing-changes-to-your-project/creating-a-commit-with-multiple-authors

cc @bskim45

[mingrammer]

Sure! I'll change the commit message.

[mingrammer]

@feluelle @bskim45 I changed the commit message with the existing ticket number and added bskim45 as co-author.

[feluelle]

Thank you guys for being so collaborative :) I can merge it when ✔️

[codecov-io]

Codecov Report

Merging #6418 into master will decrease coverage by 0.34%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #6418      +/-   ##
==========================================
- Coverage   84.25%   83.91%   -0.35%     
==========================================
  Files         627      627              
  Lines       36525    36529       +4     
==========================================
- Hits        30775    30653     -122     
- Misses       5750     5876     +126

Impacted Files	Coverage Δ
airflow/operators/slack_operator.py	100% <100%> (ø)	⬆️
...irflow/contrib/operators/slack_webhook_operator.py	91.3% <100%> (+0.39%)	⬆️
...ow/contrib/example_dags/example_qubole_operator.py	32.5% <0%> (-57.5%)	⬇️
airflow/kubernetes/volume_mount.py	44.44% <0%> (-55.56%)	⬇️
airflow/kubernetes/volume.py	52.94% <0%> (-47.06%)	⬇️
airflow/kubernetes/pod_launcher.py	45.25% <0%> (-46.72%)	⬇️
airflow/kubernetes/kube_client.py	33.33% <0%> (-41.67%)	⬇️
...rflow/contrib/operators/kubernetes_pod_operator.py	70.14% <0%> (-28.36%)	⬇️
airflow/gcp/example_dags/example_dataflow.py	93.33% <0%> (-6.67%)	⬇️
airflow/models/baseoperator.py	95.93% <0%> (ø)	⬆️
... and 5 more

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