CLEANUP: tools/cbor: rename cbor_encode_ctx struct members

CLEANUP: tools/cbor: rename cbor_encode_ctx struct members

Rename e_byte_fct to e_fct_byte and e_fct_byte_ctx to e_fct_ctx, and
adjust some comments to make it clear that e_fct_ctx is here to provide
additional user-ctx to the custom cbor encode function pointers.

For now, only e_fct_byte function may be provided, but we could imagine
having e_fct_int{16,32,64}() one day to speed up the encoding when we
know we can encode multiple bytes at a time, but for now it's not worth
the hassle.