Normalize the gutenberg.settings config and support image sizes filtering

Problem/Motivation

Everytime the relevant entity type for a Gutenberg enabled entity is save, it automatically adds in new changes in the config for every new block instance it finds, which can be annoying if you have loads of derivative fields e.g. block content blocks.

Steps to reproduce

Enable Gutenberg on your Node entity type, save then export the configuration.

Then introduce a new block type instance, e.g. an exposed filter block or a new content block.

Then once you resave the Node entity type, it'll automatically add those new block instances into the config like this in your config diff:

+  'block_content:f5c762e6-962a-42cf-841b-611e5ee40204': 0
+  'block_content:ced96c38-d1a0-4d34-ba1f-946de1291938': 0
+  'block_content:b5bfb9e5-8a8f-4db3-a343-7e3d8522feb6': 0
+  'block_content:59437793-d207-4182-bf96-52fa8c04755a': 0

Proposed resolution

Remove irrelevant config on save, as well as a post update hook to clean up the existing config.

We should ideally store these inside the individual Node entity's third_party_settings, that way it can be modelled consistently in the config schema, just like it was previously done in #3163200: Make Gutenberg entity-type-agnostic, but that's work to be in a future major release since it's a big change to the current data structure.

Remaining tasks

Provide update hook and MR.

User interface changes

N/A

API changes

N/A

Data model changes

Inactive blocks will no longer be exported in the config.