Add fuzz testing for notifiers

[pjbgf]

Following-up from the security audit, I reviewed the initial Fuzz PR and created this PR that supersedes it.

Main changes:

• Create fuzzers for all notifiers that required no code change to support fuzzing.
• Structure the fuzz implementation to be closer to what fuzz testing might look like once golang 1.18 built-in fuzz feature is released.
• Add Makefile target to enable smoke testing fuzzers locally.
• Add github workflow to test and upload crash results.

[stefanprodan]

LGTM

Thanks @pjbgf