Adds support for BYO Managed Identity & staged deployment sample

[Gordonby]

PR Summary

Closes #536

1. Adds support for bringing your own cluster identity. Useful for when the deploying user has insufficient permissions to create role assignments in the subscription.
2. Includes a sample for a staged deployment to fully illustrate the roles and responsibilities of a staged "byo" deployment.

PR Checklist

• PR has a meaningful title
• Summarized changes
• This PR is ready to merge and is not Work in Progress
• Link to a filed issue

[Gordonby]

As discussed on the triage call, the sample doc in the PR does definitely take the happy path.

I might split this PR into 2.

1. Adding Byo Identity in the bicep
2. The sample directory

[Gordonby]

@khowling and I just walked through PR.
I'll run the 3 deployment tests for each identity option, then we're good to approve/merge

[Gordonby]

Type	Command	Result	Pic
System	az deployment group create -g gordons -f bicep/main.bicep -p resourceName=sysId JustUseSystemPool=true	Success
Create UAI	az deployment group create -g gordons -f bicep/main.bicep -p resourceName=createuai JustUseSystemPool=true custom_vnet=true	Success
BYO UAI	az identity create -n id-byo-id-gords -g gordons; az deployment group create -g gordons -f bicep/main.bicep -p resourceName=byouai JustUseSystemPool=true custom_vnet=true byoUaiName=id-byo-id-gords	Success

@khowling - All 3 deployments work great.