asn1: Add support for IA5String

[facutuesca]

This adds support for encoding/decoding IA5String.

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

@asn1.sequence
class MySeq:
    field: asn1.IA5String

object = MySeq(field=asn1.IA5String("A string"))
encoded = encode_der(object)

decoded = decode_der(MySeq, encoded)
assert decoded.field.as_str() == "A string"

Part of #12283

[alex]

slightly unfortunate that we end up doing 2x validation, once on construction and then again on serialization.

can you put a TODO somewhere (either on the issue or as a follow up PR with appropriate comments) for us to look for a way to improve that perf?

[alex]

Another follow up: can you have us use to_str() on py310+ and to_cow() only on older Pythons? We do 3.11 wheels, so they'll get the perf win.