rsaSsaPssParams
Parses parameters as RSASSA-PSS parameters if this identifier uses the id-RSASSA-PSS OID.
This helper models RFC 4055, section 3.1 without making X509AlgorithmIdentifier itself enforce algorithm-specific parameter schemas during generic DER parsing.
Return
null if this algorithm is nor RSA_SSA_PSS
Throws
if this algorithm is RSA_SSA_PSS has no parameters, or the parameter element is not a valid RSASSA-PSS-params SEQUENCE.
From Swift/Objective-C use the throwing rsaSsaPssParams() accessor (exported as a static rsaSsaPssParams(_:), since value classes are not bridged as Objective-C types).