Add support for writing unsigned numbers to CBOR

[Radiokot]

Hey guys,

In a project I use Jackson CBOR dataformat for, the CBOR encoded in Kotlin is decoded by software expecting uint64. This PR implements support for writing unsigned numbers (long, int) in custom serializers, so values negative in Java do not end up negative in the CBOR. (https://datatracker.ietf.org/doc/html/rfc7049#appendix-A, https://datatracker.ietf.org/doc/html/rfc7049#appendix-B)

I've added generator methods and tests for them, but I am currently not sure what to do with the parser. In fact, an unsigned number can be parsed as BigInteger which is then converted to long via BigInteger::toLong, but it lacks the boundary check.

Currently, this can be used with custom serializers like this:

class UnsignedLongSerializer : JsonSerializer<Long>() {
    override fun serialize(value: Long, generator: JsonGenerator, serializerProvider: SerializerProvider) =
        (generator as CBORGenerator).writeNumberUnsigned(value)
}

class UnsignedLongDeserializer : JsonDeserializer<Long>() {
    override fun deserialize(parser: JsonParser, deserializationContext: DeserializationContext): Long =
        (parser as CBORParser).bigIntegerValue.toLong()
}

data class Account
@JsonCreator
constructor(
    val name: String,
    @JsonSerialize(using = UnsignedLongSerializer::class)
    @JsonDeserialize(using = UnsignedLongDeserializer::class)
    val balance: Long,
)

Do you think if support for unsigned numbers is needed in Jackson? If so, what should I add to proceed?

Thanks.

[cowtowncoder]

On reader side: not 100% sure. Was about to suggest caller has to deal with it but... haven't thought it through really.

As to support needed/useful; I think what you propose on writing makes sense as sort of "least we can do" approach.

[cowtowncoder]

Ok, looks good to me.

Just one process thing before I merge this PR: unless we have gotten one already (I don't think so but if we have LMK), we'll need a CLA; doc is here:

https://github.com/FasterXML/jackson/blob/master/contributor-agreement.pdf

it only needs to be sent once before the first contribution. The usual way is to print it, fill & sign, scan/photo, email to cla at fasterxml dot com.
Once we have it I can merge this PR & it's good for any other PRs for all Jackson repos.

Thank you again, looking forward to merging!