chore: Use optional arguments where appropriate

[malandis]

This PR refactors the code to use optional arguments where appropriate. Many methods were extraneous because of this, so we can now delete a lot of repeated code, eg:

public CacheSetResponse Set(string cacheName, string key, string value, uint ttlSeconds) {}
// ...
public CacheSetResponse Set(string cacheName, string key, string value) {}

becomes

public CacheSetResponse Set(string cacheName, string key, string value, uint? ttlSeconds = null)

Where the default for ttlSeconds is set in one place.