feat: implement local contexts

[d4rky-pl]

Fixes #247, this is my attempt at the local context problem. Please let me know if there are any further tests you'd like me to add to check if this implementation is solid enough to handle all the possible use cases (thread safety?)

[d4rky-pl]

I was trying to implement a thread safety specs but realized it doesn't make sense:

• For default Honeybadger agent instance or one created by Honeybadger::Agent the context manager is already implemented as a thread variable (Thread.current) so the state here is not shared between the threads
• For Honeybadger::Agent created with local_context: true the context manager is already not thread safe as every thread sharing the agent can modify the context of the other threads

Please let me know if there's anything I can do to fast track releasing this feature. This would help a lot in being able to use tags efficiently without resorting to either manually clearing them or manually handling exceptions within a block.