libicon: Add tool to encode/decode custom float format

I've encoded and decoded the HVIF format's custom three byte float
format by hand several times by now. This adds a python tool to
automatically encode and decode these.

Change-Id: I92cc1697a0b4fc3b55bb20a5bbcf6e93e3c0738b
Reviewed-on: https://review.haiku-os.org/c/haiku/+/6448
Reviewed-by: waddlesplash <waddlesplash@gmail.com>