az network front-door create should allow user defined names

- If the issue is to do with Azure CLI 2.0 in-particular, create an issue here at [Azure/azure-cli](https://github.com/Azure/azure-cli/issues)

### Extension name (the extension in question)
front-door

### Description of issue (in as much detail as possible)
Currently, the command used for creating a new front-door uses a number of default names for the given sub components (frontend/backend pool/routing rules etc)

https://github.com/Azure/azure-cli-extensions/blob/4428fb63c6d01876247c6fa8a9ac5e8b5cb07a64/src/front-door/azext_front_door/custom.py#L147

It would very much appreciated if this could be extended so that users can define these values as attempting to 'update' these name values requires all sorts of work arounds given you don't have an easy way to simply update the names without breaking dependancies!

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