LibWeb: Implement integrity-metadata part of fetch algorithm

Specifically, this makes `<link>` elements with an `integrity` attribute
actually work. Previously, we would load their resource, and then drop
it on the floor without actually using it.

The Subresource Integrity code is in `LibWeb/SRI`, since SRI is the name
of the recommendation spec: https://www.w3.org/TR/SRI/

However, the Fetch spec links to the editor's draft, which varies
significantly from the recommendation, and so that is what the code is
based on and what the spec comments link to:
https://w3c.github.io/webappsec-subresource-integrity/

Fixes #18408

Userland/Libraries/LibWeb/CMakeLists.txt

@@ -470,6 +470,7 @@ set(SOURCES
     RequestIdleCallback/IdleDeadline.cpp
     ResizeObserver/ResizeObserver.cpp
     SecureContexts/AbstractOperations.cpp
+    SRI/SRI.cpp
     Streams/AbstractOperations.cpp
     Streams/ReadableByteStreamController.cpp
     Streams/ReadableStream.cpp