Added multi-edit support to urp asset

[Nzollner]

Purpose of this PR

The purpose of this PR is to add multi-edit to the universal render pipeline asset, described in this fogbugz case.

This is only a UI change and because of that, I have not started any testing since there should be no changes to the result.

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Testing status

I have tested the multi-edit on my laptop(Windows 10, using dx11).
I tested the multi-edit by checking if I could change the values and if the values change affected the view and if the actions were undoable which they were.

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Comments to reviewers

When you test this check if the values work as expected and if the changes correspond to the visual changes which should be applied with the changed values.

[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://yamato.cds.internal.unity3d.com/jobs/902-Graphics
Search for your PR branch using the sidebar on the left, then add the following segment(s) to the end of the URL (you may need multiple tabs depending on how many packages you change)

URP
/.yamato%252Fall-urp.yml%2523PR_URP_trunk
With changes to URP packages, you should also run
/.yamato%252Fall-lightmapper.yml%2523PR_LightMapper_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.

[alex-vazquez]

Looks good to me

[martint-unity]

LGTM