[AIRFLOW-6939] Executor configuration via import path

[mik-laj]

I want to use it with MockExecutor. MockExecutor is helpful for performance tests.

---

Issue link: AIRFLOW-6939

Make sure to mark the boxes below before creating PR: [x]

• Description above provides context of the change
• Commit message/PR title starts with [AIRFLOW-NNNN]. AIRFLOW-NNNN = JIRA ID^*
• Unit tests coverage for changes (not needed for documentation changes)
• Commits follow "How to write a good git commit message"
• Relevant documentation is updated including usage instructions.
• I will engage committers as explained in Contribution Workflow Example.

^* For document-only changes commit message can start with [AIRFLOW-XXXX].

---

In case of fundamental code change, Airflow Improvement Proposal (AIP) is needed.
In case of a new dependency, check compliance with the ASF 3rd Party License Policy.
In case of backwards incompatible changes please leave a note in UPDATING.md.
Read the Pull Request Guidelines for more information.

[codecov-io]

Codecov Report

Merging #7563 into master will decrease coverage by 0.25%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #7563      +/-   ##
==========================================
- Coverage   86.81%   86.56%   -0.26%     
==========================================
  Files         896      896              
  Lines       42656    42657       +1     
==========================================
- Hits        37033    36926     -107     
- Misses       5623     5731     +108

Impacted Files	Coverage Δ
airflow/executors/executor_loader.py	100% <100%> (+21.05%)	⬆️
airflow/kubernetes/volume_mount.py	44.44% <0%> (-55.56%)	⬇️
airflow/kubernetes/volume.py	52.94% <0%> (-47.06%)	⬇️
airflow/kubernetes/pod_launcher.py	47.18% <0%> (-45.08%)	⬇️
...viders/cncf/kubernetes/operators/kubernetes_pod.py	69.69% <0%> (-25.26%)	⬇️
airflow/kubernetes/refresh_config.py	50.98% <0%> (-23.53%)	⬇️
airflow/plugins_manager.py	89.13% <0%> (+2.17%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 835d859...c865908. Read the comment docs.