Explose configurable key in KeychainPersistence on OSX

[mattmshell]

build_encrypted_persistence doesn't provide sufficient control over the key name which leads to confusion. The value 'msal-extensions' is hardcoded in KeychainPersistence:

[rayluo]

The build_encrypted_persistence() was meant to be a cross-platform helper so it contains no platform-specific parameters. Besides, the value "msal-extensions" was just a default value; it is customizable. You could copy build_encrypted_persistence()'s implementation and change the way it initializes KeychainPersistence.