Add asset id field icon display

[AlexanderVorobyov]

• Add AttributeEditorGUIUtility.BeginAssetFieldIcon() and EndAssetFieldIcon() methods to draw icon button inside of object field.
• Add AttributeEditorGUIUtility.BeginAssetFieldIconReference() and EndAssetFieldIconReference() methods to draw reference icon button inside of object field, used to provide additional information for field implemented as reference value.
• Add AssetFieldIconScope and AssetFieldIconReferenceScope structures as disposable scopes to draw field icon buttons.
• Add Reference information and copy button for File Id, Asset Id, Asset Guid and Resource Path attributed object select fields.

[AlexanderVorobyov]

Create branch from main

issue-284-add-asset-id-field-icon-display