lib: json: Add encoding support

Serializing an object in JSON is quite tricky to do by hand, and with
an array of descriptor structs, there's enough information to do that
programatically.

The encoder takes a callback function, so that one can be written to
write bytes to, for instance, a struct net_buf.  This way, there's no
guesswork to determine the buffer size, reducing the possibility of
overflowing the stack.

Jira: ZEP-1607
Change-Id: I5ccf1012e46c1db32fcfdf2ecee4a1ef44c927d5
Signed-off-by: Leandro Pereira <leandro.pereira@intel.com>