Allow configured default name for functionapp commands

**Is your feature request related to a problem? Please describe.**
`webapp` commands can access [configured defaults](https://docs.microsoft.com/en-us/cli/azure/reference-index?view=azure-cli-latest#az-configure): `group` and `web`. `functionapp` commands don't have an equivalent configurable default for the `name`
parameter.

**Describe the solution you'd like**
I'd like to add a configurable default for function app name: either reuse the `web` default, or add a `functionapp` default. Seems like it would be a pretty simple fix involving updating the [`name` arg_type](https://github.com/Azure/azure-cli/blob/0ce0448c9cbe73faa26c56ffcc9bc10bbf3b30f2/src/azure-cli/azure/cli/command_modules/appservice/_params.py#L44).

**Describe alternatives you've considered**
Typing more commands.

**Additional context**