Adding Message and Serialization fuzzers to libpdx

Adds two new fuzzers to libpdx for the Message and Serialization functionalities.
Test: Ran on a Pixel3a device for about 1 million iterations each, getting about 3% code coverage between 1-10k iterations per second.

Signed-off-by: Corbin Souffrant <corbin.souffrant@leviathansecurity.com>
Change-Id: I33cc67f71763bcbe2ee293964a693661e169c427