Stephaneey/podidentity

[stephaneey]

The purpose of this PR is to make a more granular use of managed pod identities to let the scaler use more than one single identity. Workloads can specify which identity should be used by the scaler to watch the corresponding Azure service. Issue #2656

Checklist

• Commits are signed with Developer Certificate of Origin (DCO - [learn more] => ok (https://github.com/kedacore/keda/blob/main/CONTRIBUTING.md#developer-certificate-of-origin-signing-your-work))
• Tests have been added => test have been modified
• A PR is opened to update our Helm chart (repo) (if applicable, ie. when deployment manifests are modified) => N/A
• A PR is opened to update the documentation on (repo) (if applicable) => Nope, let me know
• Changelog has been updated and is aligned with our [changelog requirements] => Yep

[stephaneey]

Did not run the tests locally, I will try to fix them first and create another PR once everything is ok.