Testing az cli extension

The steps for how to test an az cli extension are split over multiple repos:
- https://github.com/Azure/azure-cli/blob/dev/doc/extensions/authoring.md
- https://github.com/Azure/azure-cli-dev-tools/blob/dev/README.md
- https://github.com/Azure/azure-cli-extensions/blob/main/README.md

This makes it really difficult for a new developer to onboard quickly.

I wanted to open this issue to discuss the possibility of adding some scripts that developers can run in a single command to get bootstrapped, to create a new extension, and to run tests. Samples taken from #5944

```
# bootstrap
git clone https://www.github.com/azure/azure-cli
git clone https://www.github.com/azure/azure-cli-extensions
python3 -m venv azcli-extension-env
source azcli-extension-env/bin/activate
pip install azdev
azdev setup --cli ./azure-cli --repo ./azure-cli-extensions
```

```
# test
cd azure-cli-extensions
azdev setup -r . -e
azdev test
```