Add change_output_type to StreamDeserializer

[dr-bonez]

Summary

Adds a change_output_type method to StreamDeserializer that lets you switch the deserialization target type without rebuilding the deserializer or losing your position in the stream.

Motivation

When reading a CBOR stream that contains a mix of different types (e.g. a header followed by a sequence of records), you currently have to create a new StreamDeserializer for each type you want to read. Since the T parameter on StreamDeserializer is only used as a PhantomData marker, there's no reason you shouldn't be able to swap it out on the fly.

This comes up in practice when you have a framed protocol or file format where different sections of the stream have different schemas. Without this, you end up having to drop down to serde_cbor_2::Value and convert manually, or do awkward workarounds with the reader.