Fuzzing: Add fuzzer for API definitions

[AdamKorcz]

Adds a fuzzer for the marshaling and unmarshaling routines of the API definitions.

An internal function is added for every single API definition and a single fuzzer calls these internal functions.

FuzzApiMarshaling implements the fuzzer. checkData implements a helper that is used in every internal function. From line 365 the internal functions of the API definition are found. The only way they differ from one another is in the API definition they target. Each function targets a single Containerd API.