asn1: Add support for BIT STRING

[facutuesca]

This PR adds support for BIT STRING to the ASN.1 API

@asn1.sequence
class Example
  field: asn1.BitString

bt = asn1.BitString(data=b"\x01\x10", padding_bits=2)

object = Example(field=bt)
encoded = encode_der(object)

decoded = decode_der(Example, encoded)
assert decoded.field.as_bytes() == b"\x01\x10"
assert decoded.field.padding_bits() == 2

Part of #12283

[alex]

@reaperhulk does a BitString type (with API that looks like rust-asn1: as_byes() + padding_bits()) look right to you?