chore: make Clearcut logger a global singleton

[yulunz]

We will add a error logging method to ClearcutLogger in a follow-up PR. Since the error can happen anywhere in the stack, the logger instance has to be readily available (i.e. w/o passing the logger instance everywhere). This commit registers the instantiated logger as a global singleton, and makes it possible to retrieve it by a static method (ClearcutLogger.get()) wherever we need it.