[Modules] Extend Web Sites with kvrefidentity

[fblix]

Description

This PR will introduce the ability to define a specific identity to be used by the app service when authenticating to a keyvault.
Previously this missing property could lead to issues when using a system- and a user-assigned identity at the same time.

Pipeline references

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Type of Change

Please delete options that are not relevant.

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Update to documentation

Checklist

• I'm sure there are no other open Pull Requests for the same update/change
• My corresponding pipelines / checks run clean and green without any errors or warnings
• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation (readme)
• I did format my code

[eriqua]

🆗