ensure correctness of signatures in vote extenstions

[ttarsi]

Problem to Solve

k1util.Verify used in Vote.Verify and AggVote.Verify doesn't not check that the s component of the signature is in the lower half order. The openzeppelin ECDSA contract does this, but our k1util currently does not. So validators could submit invalid signatures, which would result in xsubmissions reverting.

Proposed Solution

add a check in k1util.Verify to ensure s is in the lower range, and check all vote extension are valid.

if (s > 0x7FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF5D576E7357A4501DDFE92F46681B20A0) {
   // error 
}