Enhancement: add extra encoding/decoding half precision floating point functions to public API

It's uncommon to operate half precision floating point data on application level, while such format is widely used on transport level to minimize traffic. In such case one can need API for encoding/decoding half-precision floating point data AS more common single or double. Something like this:
```c
CborError cbor_encode_float_as_half_float(CborEncoder *encoder, float value);
CborError cbor_encode_double_as_half_float(CborEncoder *encoder, double value);
CborError cbor_value_get_half_float_as_float(const CborValue *value, float *result);
CborError cbor_value_get_half_float_as_double(const CborValue *value, double *result);
// etc.
```
There is some related stuff in the private area, and I think, I can make it public.