CDRIVER-5676 implement bson_validate libfuzzer entrypoint

[spencerjackson]

This patch implements a fuzzer for bson_validate. It works be performing the following steps:

1. Implement a new entrypoint
2. Adding the entrypoint to CMakeLists
3. Adding '-fno-sanitize-recover=all' to sanitizer flags, so that UBSan faults become unrecoverable.

[kevinAlbs]

Thank you for the contribution. LGTM with removal of an unnecessary flag.

[eramongodb]

Minor suggestion; otherwise, LGTM.