Use naming convention as Portal for consumption or allow for existing plan to be passed

[kamperiadis]

Related command
az functionapp create

Is your feature request related to a problem? Please describe.

1. Currently we only allow customers to pass the region for the consumption plan and will pass that region to the backend call, which will create a regional plan for the customer. However, this could mean having several apps on the same plan.
2. We assume the value passed in --plan is not a consumption plan, which means that customers cannot use an existing consumption plan and then pass it to the az functionapp create command.

Related: #11195

Describe the solution you'd like

1. We should create the plan on the client side for the customer and follow the same naming convention used by the Portal that would ensure there is a unique consumption plan per app.
2. If customers want to pass an existing consumption plan, they should be able to do so with the --plan

Describe alternatives you've considered
Right now, customers might only resort to using ARM templates.

Additional context
N/A

[yonzhan]

Thank you for opening this issue, we will look into it.

[microsoft-github-policy-service]

Thanks for the feedback! We are routing this to the appropriate team for follow-up. cc @AnatoliB, @Francisco-Gamino, @shreyabatra4.