[21.3][URP] Backport of RTHandles Load/Store actions optimizations fix

[janice-see]

Checklist for PR maker

• Have you added a backport label (if needed)? For example, the need-backport-* label. After you backport the PR, the label changes to backported-*.
• Have you updated the changelog? Each package has a CHANGELOG.md file.
• Have you updated or added the documentation for your PR? When you add a new feature, change a property name, or change the behavior of a feature, it's best practice to include related documentation changes in the same PR. If you do add documentation, make sure to add the relevant Graphics Docs team member as a reviewer of the PR. If you are not sure which person to add, see the Docs team contacts sheet.
• Have you added a graphic test for your PR (if needed)? When you add a new feature, or discover a bug that tests don't cover, please add a graphic test.

---

Purpose of this PR

This is a backport of PR #6334 to fix reported foveation issues.
This particular fix was applied and only backported up till 2022.1. We received a request to backport it to 2021.3 as well.

---

Testing status

Verified that the foveation issues were fixed with the requester.

---

Comments to reviewers

This was a cherry-pick of the original PR, with some minor RTHandle -> RenderTargetHandle adjustments.

[github-actions]

It appears that you made a non-draft PR!
Please convert your PR to draft (button on the right side of the page).
See the PR template for more information.
Thank you!

[github-actions]

Welcome to the Unity SRP repo!
Please make sure to fill out the PR template as best you can to give reviewers as much information as possible.
If you have any questions (and you are a Unity employee) go to "#devs-renderpipe"

[ernestasKupciunas]

Approve without testing. Looks good.

[sebastienlagarde]

Hi @janice-see . You must use private unity repo for change on 21.3, this public graphics repostiory is for "read only".