[HDRP] Automatic alpha to coverage

[adrien-de-tocqueville]

Purpose of this PR

PR to reenable automatic alpha to coverage that was reverted due to a broken DXR test
This PR addtionally includes a change to workaround the issue, which has been reported here https://fogbugz.unity3d.com/f/cases/1392756/
(The bug is in shadow mapping, where removing ALPHATOMASK keyword would batch non batchable renderers, worked around by enabling gpu instancing on one material to prevent it from being batched with others)

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Testing status

see original PR + tested failing DXR test + launched yamato
HDRP_DXR fails for infratructure reasons apparently but problematic test 902 is green locally

[github-actions]

Hi! This comment will help you figure out which jobs to run before merging your PR. The suggestions are dynamic based on what files you have changed.
Link to Yamato: https://unity-ci.cds.internal.unity3d.com/project/902/
Search for your PR branch using the search bar at the top, then add the following segment(s) to the end of the URL (you may need multiple tabs depending on how many packages you change)

HDRP
/jobDefinition/.yamato%2Fall-hdrp.yml%23PR_HDRP_trunk
With changes to HDRP packages, you should also run
/jobDefinition/.yamato%2Fall-lightmapping.yml%23PR_Lightmapping_trunk

Depending on the scope of your PR, you may need to run more jobs than what has been suggested. Please speak to your lead or a Graphics SDET (#devs-graphics-automation) if you are unsure.

[github-actions]

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