Add thread safety to all koanf public methods

[musicpulpite]

This PR addresses the concerns brought up in Issue #355 and previously in Issue #153.

Since the koanf.confMap is a shared resource that could be accessed continuously from different goroutines for reading/updating config values, the solution is to acquire and release a sync.RWMutex in each public method that accesses the confMap property.

A more robust and performant solution would be to refactor the existing koanf.confMap map property and all methods in "github.com/knadh/koanf/maps" to use the inherently thread safe sync.Map structure from the standard library. This approach would remove the overhead of adding RWMutex locking to any new public methods and provide more granular read locking in the case of many concurrent read operations where the current approach locks the entire map structure for each read. The obvious downside is that it would require a much more intensive refactor.

[knadh]

Looks like the mutex is causing a deadlock: https://github.com/knadh/koanf/actions/runs/13138967660/job/36969992725?pr=339