NEW: Added custom InputBinding validation and editor UI drawing interface

[jamesmcgill]

Description

Added InputSystem.customBindingPathValidators which enables external parties to hook in custom code to check if the input binding path is valid.
This is only checked in the InputAsset editor.
Invalid binding paths will be shown with a warning icon next to them in the list.
This interface also allows external users to inject custom UI rendering code into the Bindings properties panel for any invalid InputBinding path so that detailed information about the warning can be displayed.

Changes made

Added InputSystem.customBindingPathValidators interface and support functions for the editor to call. These are backed by implementations in InputManager.
InputAsset editor is calling the support functions to check for validity
Added new support functions to DelegateHelpers

Checklist

Before review:

• Changelog entry added.
  • Explains the change in Changed, Fixed, Added sections.
  • For API change contains an example snippet and/or migration example.
  • FogBugz ticket attached, example ([case %number%](https://issuetracker.unity3d.com/issues/...)).
  • FogBugz is marked as "Resolved" with next release version correctly set.
• Tests added/changed, if applicable.
  • Functional tests Area_CanDoX, Area_CanDoX_EvenIfYIsTheCase, Area_WhenIDoX_AndYHappens_ThisIsTheResult.
  • Performance tests.
  • Integration tests.
• Docs for new/changed API's.
  • Xmldoc cross references are set correctly.
  • Added explanation how the API works.
  • Usage code examples added.
  • The manual is updated, if needed.

During merge:

• Commit message for squash-merge is prefixed with one of the list:
  • NEW: ___.
  • FIX: ___.
  • DOCS: ___.
  • CHANGE: ___.
  • RELEASE: 1.1.0-preview.3.

[Tanya-Li-Unity]

@jamesmcgill Thanks for implementing this. I made the changes in our OpenXR package side and review it together with my team. It works as expected.
(ignore my previous comments: those UI styling requests can be handled in our OpenXR package side.)

[jamesmcgill]

Thanks for the review @Tanya-Li-Unity
Merging now