Added an option to disable runtime debug window hotkey.

[adrien-de-tocqueville]

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Purpose of this PR

fix for https://fogbugz.unity3d.com/f/cases/1345783/

it is now possible to disable the debug window hotkey by doing:

DebugManager.instance.enableWindowHotkey = false;

Ideally, the hotkey should be customizable but it is a bit more involved to do since we have to support the two input systems. (Created a JIRA task: https://jira.unity3d.com/browse/HDRP-1239)

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Testing status

tested in playmode and in player build

[sebastienlagarde]

@adrien-de-tocqueville can you update the documentation so users know they can do this. Put it in the debug menu

[iM0ve]

Testing should be sufficient for the scope of the change

[github-actions]

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