[Feature Request] Make AppVersion optional

[kinbiko]

What problem are you trying to solve?

Remove the need for the AppVersion config parameter from Go v1.18 onwards.
Go 1.18 will embed version information in the binary, and presumably a binary can know its own version.

Describe how you wish this package would help you solve the problem

Make AppVersion optional, and if missing attempt to read natively as part of the one-off data gathering when initialising a notifier. Only error when no AppVersion was provided and Go couldn't read it from the build information itself.