Add ability to clone a layout builder layout field

Merge request created. Working so far across a variety of complex layouts in my initial tests.

- New "Copy layout" field processor plugin, only offered for layout builder layout type fields.
- Clones each section inside of the layout
- Clones each component inside of each section, giving each component its own UUID. UUIDs are not intended to be shared.
- If component is a layout builder-provided content block, the content block is also cloned (at the revision specified by the component). These blocks are not intended to be shared across layouts. Cloning these blocks prevents cross-talk issues between original and cloned layouts. Block clone is done content_entity_clone-style if clone settings are enabled for the block type, falling back to a core entity clone.
- Copy values plugin adjusted to not show up for layout builder layout type fields, as it is inappropriate for that field type.

Needs review and additional testing.

Attached patch represents the current state of MR8 at the time of this comment.