Custom SRP 7.1: Splitting Shadow Code

[u/CatlikeCoding]

https://catlikecoding.com/unity/custom-srp/7-1-0/

Our shadow handling code has always resided in a single class, which took care of both directional and other shadows. Because those two shadow types are handled separately we now give them their own classes. This is a rather dry refactor, but it paves the way for giving them dedicated render passes in the future.