Individual section sticky headers

[kyleve]

Allows making individual section headers sticky, a couple ways:

• By implementing isStickySectionHeader on HeaderFooterContent – this allows headers which are designed to be sticky, eg, MarketStickySectionHeader, to force this value on.
• By setting isHeaderSticky on participating SectionLayoutsValue, such as the .table layout type.

In order to support isHeaderSticky on SectionLayoutsValue, we need to be able to access the underlying type during layout – to do this, ListLayout gained three associated types:

    associatedtype ItemLayout:ItemLayoutsValue
    associatedtype HeaderFooterLayout:HeaderFooterLayoutsValue
    associatedtype SectionLayout:SectionLayoutsValue

Checklist

Please do the following before merging:

• [x] Ensure any public-facing changes are reflected in the changelog. Include them in the Main section.