[AIRFLOW-5498] Move GCP Discovery hook to core

[turbaszek]

Make sure you have checked all steps below.

Jira

• My PR addresses the following Airflow Jira issues and references them in the PR title. For example, "[AIRFLOW-XXX] My Airflow PR"
  • https://issues.apache.org/jira/browse/AIRFLOW-5498

Description

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Tests

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  4. Subject uses the imperative mood ("add", not "adding")
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  6. Body explains "what" and "why", not "how"

Documentation

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[turbaszek]

This PR will require note after #6119

[codecov-io]

Codecov Report

Merging #6121 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #6121      +/-   ##
==========================================
+ Coverage   80.12%   80.13%   +<.01%     
==========================================
  Files         607      609       +2     
  Lines       34905    34922      +17     
==========================================
+ Hits        27967    27984      +17     
  Misses       6938     6938

Impacted Files	Coverage Δ
airflow/contrib/hooks/google_discovery_api_hook.py	100% <100%> (ø)	⬆️
...low/contrib/operators/google_api_to_s3_transfer.py	100% <100%> (ø)	⬆️
airflow/operators/google_api_to_s3_transfer.py	100% <100%> (ø)
airflow/gcp/hooks/discovery_api.py	100% <100%> (ø)
airflow/jobs/scheduler_job.py	74.28% <0%> (-0.31%)	⬇️
airflow/utils/dag_processing.py	58.98% <0%> (+0.18%)	⬆️
airflow/api/common/experimental/delete_dag.py	94.28% <0%> (+5.82%)	⬆️

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