[Enhancement] Do not use custom written Optional<T> class

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• I searched in the issues and found nothing similar.

Version

repo/master

Minimal reproduce step

template <> class Optional;

What did you expect to see?

Use optional class from boost or std

What did you see instead?

Used custom written Optional class

Anything else?

At this moment pulsar-client-cpp heavily used boost and std library. I suggest to use boost::optional instead of Optional class.
Boost::optional well-tested and has similar API with std::optional (at most).
In this issue I do not suggest to increase C++ standart to C++17 (from C++11) where std::optional is available, but in the future it can be done easily.

Are you willing to submit a PR?

• I'm willing to submit a PR!