Update dynamically on post types that use the Block Editor

[mogmarsh]

Description

On post types that use the Block Editor (Gutenberg), it would be useful if the inspector reflected the Post Editor's data.

Use Case

Currently in the Block Editor, you must Save the post, then refresh to see the latest meta data.

[mogmarsh]

I've been discussing with @kevinfodness, and here's what we think this would look like:

• add a PluginSidebar to display all meta. Each meta field can be in a collapsible panel so it isn't overwhelming. If the Block Editor is active, suppress the existing metabox.
• add a custom REST endpoint to query all post meta, even the meta values that aren't exposed to the Block Editor. This MUST be locked down.
• The sidebar should use usePostMeta to get a value that has been updated by the Block Editor, and fail back to the value from the REST endpoint.

Bonus points:

• Add a button to view meta values in a modal in addition to the sidebar, so that large meta values can be viewed easier
• Add JSON Formatter - if a value is JSON, or expressed as JSON because it's a serialized array/object in PHP, pretty-print the JSON to make it easier to read

[kevinfodness]

I'd recommend splitting the implementation steps into separate issues so that we can ship something initially that swaps the metabox for a PluginSidebar that shows meta visible to/managed by the editor, then iterates on that solution by adding the hidden meta, adding an open in a modal button, and adding the JSON prettifier.