Replace assets-webpack-plugin dependency by an internal plugin, to generate entrypoints.json file

[Kocal]

Q	A
Bug fix?	no
New feature?	yes
Deprecations?	no
Issues	Fix #969
License	MIT

This PR replaces the https://github.com/ztoben/assets-webpack-plugin dependency with an internal plugin, this allows us:

• to drop a dependency, and simplify the code (we don't need 10 ~options to generate the entrypoints.json for the WebpackEncoreBundle)
• to not depend on https://github.com/waysact/webpack-subresource-integrity, which is necessary to use the integrity options of https://github.com/ztoben/assets-webpack-plugin
• to generate the integrity data after files have been emitted

Things done in this PR:

• Re-implement https://github.com/ztoben/assets-webpack-plugin functionality as closely as possible
• Ensure tests pass without any modifications
• Ensure the branch works like before on a real project (in dev/prod/dev-server, with or without integrity hashes)

Things not done in this PR:

• Integrity hashes do not work if contenthash is a query string #1269 will have to wait for another PR, I don't wanted to fix that here

[Kocal]

When you update a file while the dev-server is running, an .hot-update. file can be emitted, but we don't want to add it in the manifest.