[iOS] Fix for ItemSpacing not properly applied between the item and group header in CollectionView Horizontal LinearItemsLayout

[Tamilarasan-Paranthaman]

Root Cause of the issue

• The ItemSpacing for the section was not set in the GetInsetForSection method for LinearItemsLayout, defaulting to zero. This affected both horizontal and vertical orientations, resulting in no spacing between the group header and the item.

Description of Change

• I have implemented logic to apply item spacing between the group section and the item within GetInsetForSection for LinearItemsLayout. This change ensures that the spacing is correctly applied for both horizontal and vertical orientations, based on the specified orientation.

Issues Fixed

Fixes #25859

Tested the behaviour in the following platforms

• [x] Android
• [x] Windows
• [x] iOS
• [x] Mac

Screenshot

• Note: I have set the CollectionView background color to gray to make the differences more visually apparent. The gray areas between the items in the image represent the spaces.

1. LinearItemsLayout - Orientation="Horizontal"

Before Issue Fix	After Issue Fix

2. LinearItemsLayout - Orientation="Vertical"

Before Issue Fix	After Issue Fix

[dotnet-policy-service[bot]]

Hey there @Tamilarasan-Paranthaman! Thank you so much for your PR! Someone from the team will get assigned to your PR shortly and we'll get it reviewed.

[jfversluis]

/azp run

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