asn1: Add support for SET

[facutuesca]

This adds support for encoding/decoding SET.

here's an example of how it would be used:

@asn1.set
class MySet:
    foo: int
    bar: int

object = MySet(foo=6, bar=9)
encoded = encode_der(object)

decoded = decode_der(MySet, encoded)
assert decoded.foo == 6
assert decoded.bar == 9

The asn1.set decorator behaves similarly to asn1.sequence, except that the order of the fields is checked during encoding/decoding:

object = MySet(foo=9, bar=6)
encoded = encode_der(object)
# this raises due to the encoded value of `foo` being higher than `bar`

Part of #12283