[HDRP][Path Tracing] Improved volumetric scattering light sampling

[eturquin]

This PR addresses the following tickets, all related to volumetric scattering in path tracing:

• Noisy result when multiple point/area lights overlap https://fogbugz.unity3d.com/f/cases/1358777
• Exaggeratedly strong contribution from directional lights when the ray hits a surface afterwards https://fogbugz.unity3d.com/f/cases/1304688
• Fog color is now taken into account in path-traced volumetric scattering, albeit in a slightly different way than in its raster counterpart https://fogbugz.unity3d.com/f/cases/1346105 https://fogbugz.unity3d.com/f/cases/1358783

The new version shows better convergence speed, and fixes the directional light intensity issue.
It also uses the fog color to produce a result similar to the non-volumetric one, and then adds volumetrically scattered contributions on top of it (which is IMHO a better behaviour than what the raster version does, whose computations I couldn't make sense of).

This has been tested on the scenes of the respective tickets, and on an additional test scene which mixes all kinds of lights (see screenshot below).

[github-actions]

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[eturquin]

[sebastienlagarde]

For QA. I need to merge this PR for 21.2 but please do a review of it. thanks

[sebastienlagarde]

For QA: this PR is also part of it and merged as well, so when you validate it include this one as well: #5798