Support dynamic shadow casting and receiving

Describe the problem

The Graphics Tools/Standard shader doesn't have a shadow pass since we often avoid dynamic shadows in mixed reality due to performance constraints. Now that we support other platforms and mixed reality devices are getting more powerful it makes sense to allow the user to enable dynamic shadows if they are aware of the cost.

Describe the solution you'd like

Add directional shadow support like so: https://catlikecoding.com/unity/tutorials/custom-srp/directional-shadows/

We probably don't need to support point and spot shadows since those light sources aren't supported as is.

Describe alternatives you've considered

Stick to static shadows only.

Additional context

Related issue: https://github.com/microsoft/MixedRealityToolkit-Unity/issues/9689