webid: inline ECSignatureCreator::DecodeSignature

This method does not really belong on ECSignatureCreator - it parses a
DER-encoded ECDSA signature (whether it came from ECSignatureCreator or
not) then turns it into a packed vector of (r, s) from the signature.
It's also only used in the JWT signer code in webid. Since
ECSignatureCreator is deprecated anyway, this change removes
DecodeSignature altogether and moves its body into the webid code.

As part of that, the function (now named UnpackDERSignature) is now free
to assume that the signature is valid (since it just created the
signature itself) and to assume that r and s are well-formed (for the
same reason), so some error paths are converted to CHECKs.

Bug: 372283556
Change-Id: I0023795173b28bbe565150ccd37fb7f9a61b8ee6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6635283
Reviewed-by: Nicolás Peña <npm@chromium.org>
Reviewed-by: David Benjamin <davidben@chromium.org>
Commit-Queue: Elly FJ <ellyjones@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1474540}