feat: Allow custom app insights instrumentation key

[tbarlow12]

What did you implement:

Enable custom app insights instrumentation key

provider:
  appInsights:
    instrumentationKey: {your_key}

How did you implement it:

If an instrumentation key is added, arm template will use that value instead of the key from the generated template

Out of scope: The default template will still create an instance of app insights - should be future PR to remove that

How can we verify it:

Deploy a function app with a custom app insights instrumentation key

Todos:

Note: Run npm run test:ci to run all validation checks on proposed changes

• Ensure there are no lint errors.
  Validate via npm run lint
  Note: Some reported issues can be automatically fixed by running npm run lint:fix
• Write tests and confirm existing functionality is not broken.
  Validate via npm test
• Write documentation
• Provide verification config / commands / resources
• Enable "Allow edits from maintainers" for this PR
• Update the messages below

Is this ready for review?: YES
Is it a breaking change?: NO

[wbreza]

LGTM - I would like to see a unit test to validate this change before merging.

[wbreza]

Unit test to validate this change?