Allow encoding/decoding uint struct tag keys

[pierre-borckmans]

This PR adds support for encoding/decoding structures like:

type Item struct {
	Field100 string `msgpack:"100"`
	Field200 string `msgpack:"200"`
	Foo string `msgpack:"not_int_key"`
}

where some of the msgpack keys are actually of type uint.

A new flag is exposed to control this behaviour:

buf := new(bytes.Buffer)
enc := msgpack.NewEncoder(buf)
enc.UseUIntStructKeys(true)
err := enc.Encode(&Item{
	Field100: "foo",
	Field200: "bar",
	Foo: "baz",
})
if err != nil {
	panic(err)
}

var Item item
dec := msgpack.NewDecoder(buf)
dec.UseUIntStructKeys(true)
err = dec.Decode(&item)
if err != nil {
	panic(err)
}

The reason I need this is because I have a device sending msgpack objects over usb that are encoded in this way (with uint keys).

I added an example to cover this use case.
I tried to add docs but couldn't find where the sources for https://msgpack.uptrace.dev/guide are.