feat: add StdConfig::exists method to check if key exists

[nikollamalic]

Small addition to help check if a specific key exists in the config file.

Closes #773

[nikollamalic]

Hey @0xrusowsky friendly ping,

This is adding two new simple methods to StdConfig which check if a specific key exists inside the configuration cache.

It is really really helpful addition which helps us make the transition to using Config since get will revert when specific keys are not found.

Appreciate the work done on this and looking forward to your feedback!
Nikola, infiniFi

[DaniPopes]

Thanks!