Feature: parser function for string access by pointer

I have a few use cases where I have a CBOR stream with a rather large byte strings. Having a function to get a pointer and length to these byte string would help saving some memory as a buffer to temporarily copy the data to is not required. Something like
```
CborError cbor_value_get_byte_string(CborValue *value, uint8_t **result, size_t *len);
```
would be perfect. This could return an `CborErrorUnknownLength` when a chunked string is passed.

My main use case is a COSE library where I want to retrieve the payload without having to copy the to another buffer. Simply accessing it with a pointer is preferred to save memory on a constraint device.

I'm willing to contribute the code myself, but I need a few pointers on how to get the pointer to the actual data.