[Signature-based SRI] Convert SRIMessageSignature to blink types.

Post-enforcement of SRI Message Signatures in the network service, we'll
want to match them against integrity metadata in the renderer. This
patch starts down that path by doing type conversion busywork to expose
a response's signatures in a way that allows comparison.

This is a prerequisite to the following chain of CLs implementing the feature:

1.  [IntegrityMetadataSet] https://crrev.com/c/6054252
2.  [Parsing 'ed25519'] https://crrev.com/c/6054782
3.  [Matching/Enforcement] https://crrev.com/c/6055566

Bug: 380783670
Change-Id: Idf77984d8df0f70920152c9f653184e225135d8d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6043927
Reviewed-by: Camille Lamy <clamy@chromium.org>
Commit-Queue: Mike West <mkwst@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1390268}