oauth: Use the new and improved Sigstore redirect page

[tetsuo-cpp]

Closes #354

[tetsuo-cpp]

I've just decided to silence Ruff here as I don't think it helps readability to insert line breaks for this type of string.

[tetsuo-cpp]

Thanks @julienv3, this looks great.

[jku]

This is (mostly?) code copied from sigstore/sigstore right? Should include some authorship info in the commit as well -- it's good for tracking copyrights but also good for the authors internet fame and fortune (not everyone cares about that but it's still nice).

Adding a "Co-authored-by:" to the commit message would trigger at least github noticing the contribution, maybe that's good enough?

[tetsuo-cpp]

This is (mostly?) code copied from sigstore/sigstore right? Should include some authorship info in the commit as well -- it's good for tracking copyrights but also good for the authors internet fame and fortune (not everyone cares about that but it's still nice).

Adding a "Co-authored-by:" to the commit message would trigger at least github noticing the contribution, maybe that's good enough?

My mistake, I completely forgot about that. I'll add the "co-authored-by" to attribute the commit to @julienv3 and add a comment in the code to explain where it's from and who wrote it. How does that sound?

[woodruffw]

LGTM! Per #361 we might to eventually move this to a stand-alone template or file, since that might help with automatic syncing against cosign.