net: lwm2m: plain text: fix float formatting

Formatting a float32/64 value for plain text is broken.
Example for 32bit: val1=0 and val2=500000 is equivalent to 0.5

Current formatter was using %d.%d (%lld.%lld for 64bit) so
exported value was 0.500000 (or 0.5)

To fix this, for val2 use a zero-padded formatter for the maximum
length of each bit length (6 for 32bit and 9 for 64bit), and then
remove the zero characters at the end of the string.

Notes re: handling of val1/val2 signs:
- eliminate potential negative sign when converting val2 to avoid:
  a value like: 0.-5
- use negative val2 when val1 is 0 to fix small negative handling
  such as -0.5

Fixes: https://github.com/zephyrproject-rtos/zephyr/issues/16154



Signed-off-by: Michael Scott <mike@foundries.io>