GenerateFileFromTemplate: allow a user-specified variable prefix string

- [ ] This issue is blocking
- [ ] This issue is causing unreasonable pain

I am using the `GenerateFileFromTemplate` task to create a CMake config file for a NativeAOT library in order to make it easy for downstream projects to consume my native binary. CMake uses `${Foo}` syntax for writing variables. As a result my cmake template.in file looks like this:

```cmake
set_property(TARGET "${libArtifactName}::libs" PROPERTY IMPORTED_LOCATION "`${LIBBASEPATH}/`${NAME}`${EXT}")
```

here `libArtifactName` is a `GenerateFileFromTemplate` parameter, and `` `${NAME} `` is a cmake variable.

It would be nice if I could specify a custom sigil for template variables:

```xml

```

Then my template could look like this:

```cmake
set_property(TARGET "£{libArtifactName}::libs" PROPERTY IMPORTED_LOCATION "${LIBBASEPATH}/${NAME}${EXT}")
```