Add option to gzip file in loadFileAsBase64 function

**Is your feature request related to a problem? Please describe.**
Azure VM Custom Script extension can take a gzip-compressed base64 encoded script file as input.
Currently, while loading file as base64 we're limited by the maximum length of a string literal. If we prior compress the file with gzip we'll be able to load larger files in the template

**Describe the solution you'd like**
I'd like to have an optional parameter in `loadFileAsBase64` where I could specify if file should be compressed before converting to base64 or not. default value would be false.

Start by locating the loadFileAsBase64 function in the Bicep codebase and inspect how it currently loads and encodes files. The work is complete when an optional compression choice supports the Azure VM Custom Script extension use case while leaving the default behavior unchanged.

Written by the indexing model from the issue text.