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

**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: https://github.com/Azure/azure-cli/issues/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