Split out driver code into separate assembly

Currently, the "driver" code is interspersed throughout MGC. This makes it difficult to experiment with generating code without building a custom emitter in Typespec or using complex command line args. It would be better to have a separate driver that calls into MGC through public APIs. This would allow customers to generate using only C# code without having to build a custom emitter or use complex command line args. It would also allow customers to create custom drivers to customize certain aspects of how the library is generated - for instance, the file locations of the tspCodeModel.json and the configuration.json.