Use Azure Identity instead of connection string for authentication

[amerjusupovic]

Purpose

• Uses Azure Identity for authentication to Azure App Configuration, which is preferred over connection string for security

Does this introduce a breaking change?

[ ] Yes
[x] No

Pull Request Type

What kind of change does this Pull Request introduce?

[ ] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[x] Refactoring (no functional changes, no api changes)
[ ] Documentation content changes
[x] Other... Please describe: Security improvement

How to Test

• Use README instructions to ultimately run azd up, see that the App Service deployment runs as expected and doesn't fail due to authentication

What to Check

Verify that the following are valid

• All mentions of App Configuration connection strings are gone, replaced with Azure Identity credentials and the App Configuration endpoint

Other Information