[AIRFLOW-823] Allow specifying execution date in task_info API

[robin-miller-ow]

Dear Airflow Maintainers,

Please accept this PR that addresses the following issues:

• https://issues.apache.org/jira/browse/AIRFLOW-823

Testing Done:

• Unittests added to cover this code change.

[bolkedebruin]

@robin-miller-ow I think this is the wrong approach to make the change. A DAG does not have an execution date, but a DagRun does. A Task does not have an execution_date but a TaskInstance does by inheriting it from the DagRun.

So I would expect:
/dags/<string:dag_id>/<datetime:execution_date>/tasks/<string:task_id>

to return the information you are looking for.

[robin-miller-ow]

@bolkedebruin That seems inconsistent to me. When I added a date into the tigger dag API, you said it should be sent in the data, not the URL, so I presume we'd still want to do that. If this is not the right URL, then based on the trigger dag URL, would:
/dags/<string:dag_id>/dag_runs/task_instance/<string:task_id>
be more consistent? Possibly:
/dags/<string:dag_id>/dag_runs/<datetime:execution_date>/task_instance/<string:task_id>
but I would then expect the be posting to
/dags/<string:dag_id>/dag_runs/<datetime:execution_date>
to create a dag run at a specific time. Thoughts?

[bolkedebruin]

@robin-miller-ow that is actually unrelated: GET vs POST. So no that is not more consistent

[codecov-io]

Codecov Report

Merging #2045 into master will increase coverage by 0.22%.
The diff coverage is 96.72%.

@@            Coverage Diff             @@
##           master    #2045      +/-   ##
==========================================
+ Coverage   66.94%   67.16%   +0.22%     
==========================================
  Files         140      142       +2     
  Lines       10635    10688      +53     
==========================================
+ Hits         7120     7179      +59     
+ Misses       3515     3509       -6

Impacted Files	Coverage Δ
airflow/www/api/experimental/endpoints.py	97.4% <100%> (+0.73%)	✅
airflow/api/common/experimental/get_task.py	100% <100%> (ø)
...rflow/api/common/experimental/get_task_instance.py	90.9% <90.9%> (ø)
airflow/models.py	86.31% <ø> (+0.15%)	✅
airflow/jobs.py	73.01% <ø> (+0.52%)	✅

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[robin-miller-ow]

@bolkedebruin I think this is now as you would like. Please let me know if there are any further issues.

[bolkedebruin]

LGTM