Provide access to underlying encoded buffers

[BrianSipos]

I have a need to access the original buffer from an associated QCBOREncodeContext and QCBORDecodeContext and would prefer to not need to pass around the context and its associated buffer. Instead it would be convenient to be able to access the buffer from an active context. I am currently using the 'private' fields of the context to get the buffer but would rather use a public, stable, and documented function to get access.

[laurencelundblade]

Makes sense given some of the other things you've asked for.