ArmParser is missing functionality for ARM functions

[kilasuit]

Looks like we are missing the ability to extract ARM Functions from an ARM template in the following code

https://github.com/PowerShell/PSArm/blob/f80b2f86573fbe5b1db1ad21c9e6f328308e6045/src/Serialization/ArmParser.cs#L113-L158

[rjmholt]

Yeah ARM functions aren't supported yet in any part of PSArm, since they're not quite as helpful on the PowerShell side. It's likely something that won't be prioritised soon, but I'm sure we'd accept a contribution for it.