Cache block item constructors for block based editors

[kjac]

Prerequisites

• I have added steps to test this contribution in the description below

Description

When block based editors construct their published values, they use a whole lot of reflection and on-the-fly IL generated types to construct their contained block items. In short, they activate a lot of what's going on in ReflectionUtilities.

Unfortunately the reflected results are not cached between requests, even though they can never differ as they are type bound. In fact, the results are not even cached between block based editors, so if two block based editors exist on the same page one or more of the same block types, the reflection is repeated for each editor. This also even applies to nested block editors.

The block editors do cache their published values, but all this reflection will still happen for each "first page render" (and after a page publish). The result is a fair amount of CPU time spent churning reflection:

This PR aims to cache as much of this reflection as possible, so we don't have to do all of this reflection over and over.

Since block items are typed to the individual block based editors, caches must be introduced and isolated for each of them (BlockListItem<TContent, TSettings>, BlockGridItem<TContent, TSettings> and RichTextBlockItem<TContent, TSettings> respectively).

Testing this PR

To test this PR, we must ensure that the Block List, the Block Grid and the Rich Text Editor all continue to work the same for their contained blocks. This means they must work:

• With ModelsBuilder disabled having no generated models built into the site.
• With ModelsBuilder disabled having some generated models built into the site.
• With ModelsBuilder disabled having all generated models built into the site.
• With ModelsBuilder enabled in all modes.
• With ModelsBuilder running "SourceCode" modes having no generated models built into the site.
• With ModelsBuilder running "SourceCode" modes having some generated models built into the site.
• With ModelsBuilder running "SourceCode" modes having all generated models built into the site.

...and probably a few other permutations.

It might be best to reach out to @kjac for testing 😆

[Migaroez]

✅ Looks good
✅ Pair testing seems to cover everything.

[Nuklon]

This isn't thread safe, see #15421