feat: deduplicate assets by hash

[dkarlovi]

Q	A
Bug fix?	yes
New feature?	yes
BC breaks?	yes
Deprecations?	no
Fixed tickets	#349, #323

This revamps the Asset URL interpreter:

1. it no longer does a HEAD request each time just to get the filename by default, you need to opt-in to that (BC break), AssetUrlInterpreter always does HTTP requests, even if already available #349
2. it allows deduplicating assets by their MD5 checksum, not just the URL (for example, have a single asset importing from multiple URLs, it gets reused because the checksums match AND the second URL is added to the URLs we check against, it will be deduped by URL next time

[dpfaffenbauer]

can you check the static tests?

[dkarlovi]

@dpfaffenbauer done, ready for review.