SHA library

[redboltz]

Now, async_mqtt is using picosha2.h.
https://github.com/redboltz/async_mqtt/blob/main/include/async_mqtt/external/picosha2.h

A part of broker code use it. https://github.com/redboltz/async_mqtt/blob/main/include/async_mqtt/broker/security.hpp
It is for sha256 password digest.

async_mqtt/include/async_mqtt/broker/security.hpp

Lines 76 to 83 in 8d3ef84

	struct authentication {
	enum class method {
	sha256,
	plain_password,
	client_cert,
	anonymous,
	unauthenticated
	};

If I propose async_mqtt to the Boost Libraries, this external dependency should be solved.

[redboltz]

Moved to discussion #93.