Extract pending vkjson_generator utility functions into separate file

This CL contains only movement of code from one file to another, and is continuation of https://googleplex-android-review.git.corp.google.com/c/platform/frameworks/native/+/33070222

These functions weren't planned for extraction initially, given their one time usage. However, while developing unit tests for the generator, it was realized that they involve enough complexity and edge cases to merit their own unit tests (following in another CL)

Moving forward, vkjson_generator should contain only the core functions (gen_h, gen_cc, and gen_instance_cc), while all supporting utility functions should reside within vkjson_gen_util. This improves code readability and separation of concerns, apart from enabling unit test mocks.

Bug: b/409448422
Flag: NONE infeasible
Test: No changes to vkjson files after re-generation

Change-Id: I95c577005b555399eda8b4bb71307ab36e7621eb