Give more flexibility to the generators in how they create files

[ms14981]

Currently, the main forge project uses _tags to (in some cases) split the ApiClient file into multiple Handlebars template files. This works fine, but enforces which files are split, and which aren't.

This has led to the issue in the Java generator where we want to split ApiModel into several files, rather than having multiple classes under this one file name. Having this is not idiomatic in Java.

This issue is to change the main forge and its generators to give responsibility for splitting template files into multiple ones to the generators, and not making that decision in the main forge.

Currently, if we want to split the ApiModel into multiple files without changing the code for all generators, we need to add a hacky post-processing step for the Java.