This PR aims to polish the shell scripts used in aks pr check-in pipeline. The main changes are as follows；

• Update all shell scripts, add required variable checks, set bash options in a clearer way
• Update the way to forcibly update the azure-cli command index to ensure that the aks-preview command has been loaded before performing tests
• Define the trigger method in the pipeline yaml file
• Modify the implementation of AKSCustomResourceGroupPreparer to inherit (same as azure-cli)

General Guidelines

• Have you run azdev style <YOUR_EXT> locally? (pip install azdev required)
• Have you run python scripts/ci/test_index.py -q locally?

For new extensions:

• My extension description/summary conforms to the Extension Summary Guidelines.

About Extension Publish

There is a pipeline to automatically build, upload and publish extension wheels.
Once your PR is merged into master branch, a new PR will be created to update src/index.json automatically.
The precondition is to put your code inside this repo and upgrade the version in the PR but do not modify src/index.json.