Separated I/O independent (as known as Sans-I/O) protocol part.

[redboltz]

The protocol part can be used from not only Boost.Asio but also other I/O mechanism including OS-less enviromnemt.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 92.61441% with 163 lines in your changes missing coverage. Please review.

Project coverage is 96.18%. Comparing base (5243283) to head (d08d2b0).

Files with missing lines	Patch %	Lines
...clude/async_mqtt/protocol/impl/connection_impl.ipp	94.84%	47 Missing ⚠️
include/async_mqtt/impl/endpoint_impl.ipp	78.30%	41 Missing ⚠️
...clude/async_mqtt/protocol/impl/connection_send.hpp	94.26%	21 Missing ⚠️
include/async_mqtt/impl/endpoint_send.hpp	89.13%	10 Missing ⚠️
include/async_mqtt/protocol/event_timer.hpp	50.00%	10 Missing ⚠️
include/async_mqtt/protocol/impl/timer_impl.ipp	0.00%	10 Missing ⚠️
include/async_mqtt/impl/endpoint_recv.hpp	94.89%	7 Missing ⚠️
...t/predefined_layer/customized_websocket_stream.hpp	93.93%	4 Missing ⚠️
..._mqtt/predefined_layer/customized_basic_stream.hpp	94.33%	3 Missing ⚠️
...nc_mqtt/predefined_layer/customized_ssl_stream.hpp	93.87%	3 Missing ⚠️
... and 5 more

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #374      +/-   ##
==========================================
- Coverage   97.34%   96.18%   -1.16%     
==========================================
  Files         154      161       +7     
  Lines        9474    10095     +621     
==========================================
+ Hits         9222     9710     +488     
- Misses        252      385     +133     

Files with missing lines	Coverage Δ
include/async_mqtt/client.hpp	100.00% <ø> (ø)
include/async_mqtt/detail/client_impl.hpp	100.00% <ø> (ø)
include/async_mqtt/detail/endpoint_impl.hpp	100.00% <ø> (ø)
...clude/async_mqtt/impl/buffer_to_packet_variant.ipp	97.77% <100.00%> (ø)
include/async_mqtt/impl/client_auth.hpp	100.00% <100.00%> (ø)
include/async_mqtt/impl/client_close.hpp	100.00% <ø> (ø)
include/async_mqtt/impl/client_disconnect.hpp	98.38% <100.00%> (ø)
include/async_mqtt/impl/client_impl.hpp	100.00% <100.00%> (ø)
include/async_mqtt/impl/client_impl.ipp	100.00% <100.00%> (ø)
include/async_mqtt/impl/client_publish.hpp	93.13% <100.00%> (ø)
... and 68 more

... and 6 files with indirect coverage changes

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