Isolate algorithm-specific foo properly

[iaik-jheher]

There are some very specific helpers polluting general classes.

X509SignatureValue's RS foo should be removed. If we want to, we could create a data class that models this (as ECSignatureValue) and then move the foo to there (comparable to RsaSsaPssParams, see next paragraph) but it might be unnecessary.

X509AlgorithmIdentifier's rsaSsaPssParams member should go. It is far too specific for this generic location. If we want, we can keep it as an extension property in RsaSsaPssParams's companion.

[iaik-jheher]

If we choose to create a data class for the RS foo, we would draw from RFC5480 page 17:

ECDSA-Sig-Value ::= SEQUENCE {
     r  INTEGER,
     s  INTEGER 
}