Add overloading for user-defined functions

^{Originally posted by **pmalarme** May 28, 2024}
User-defined function is an amazing feature of Bicep. It makes it easy to implement naming convention across all Bicep modules for example.

Function overloading would lead to a cleaner like I could write a function. For example, I'm implementing a function to get / generate the name of a resource. I put some parameters that could be null:
```bicep
func getResourceName(resourceType resourceTypeType, workload string, env string, region string, instance string?) => ...
```
If I don't use the instance, I need to call it like this:
```bicep
param resourceGroupName string = getResourceName('resourceGroup', 'my-workload', 'dev', 'gwc', null)
```

If I could overload function, I would have defined 2 functions:
```bicep
func getResourceName(resourceType resourceTypeType, workload string, env string, region string, instance string?) => ...

func (resourceType resourceTypeType, workload string, env string, region string) => ...
```

Therefore I can call now this function like this:
```bicep
param resourceGroupName string = getResourceName('resourceGroup', 'my-workload', 'dev', 'gwc')
```