Allow the use of utcNow() in bicepparam files

**Is your feature request related to a problem? Please describe.**
Several Azure resources become immutable upon deployment, so having their name set automatically to include its deploy date makes redeploy easier. Bicep allows for utcNow() in templates as the default value for parameters, but it cannot be used in bicepparam files to automatically generate a name in the same way that this can be accomplished in bicep itself.

**Describe the solution you'd like**
The ability to use the following in a bicepparam file:
```
param imageTemplate: {
name: 'imgtemplate-${utcNow('yyMMdd')}'
...
}
```