Supported default completion token.

[redboltz]

In order to avoid function name conflict, all async functions have async_ prefix now.

Applied async function rename for all callers.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 85.36585% with 12 lines in your changes are missing coverage. Please review.

Project coverage is 89.20%. Comparing base (7f9bb0a) to head (5a0b313).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #238      +/-   ##
==========================================
- Coverage   89.22%   89.20%   -0.02%     
==========================================
  Files         121      121              
  Lines        7988     7988              
==========================================
- Hits         7127     7126       -1     
- Misses        861      862       +1     

Files	Coverage Δ
include/async_mqtt/client.hpp	100.00% <ø> (ø)
include/async_mqtt/endpoint.hpp	100.00% <ø> (ø)
...sync_mqtt/impl/client_acquire_unique_packet_id.hpp	100.00% <100.00%> (ø)
...mpl/client_acquire_unique_packet_id_wait_until.hpp	100.00% <100.00%> (ø)
include/async_mqtt/impl/client_close.hpp	100.00% <100.00%> (ø)
include/async_mqtt/impl/client_disconnect.hpp	95.00% <100.00%> (ø)
include/async_mqtt/impl/client_impl.hpp	91.12% <100.00%> (-0.81%)	:arrow_down:
include/async_mqtt/impl/client_publish.hpp	85.71% <100.00%> (ø)
include/async_mqtt/impl/client_start.hpp	80.00% <100.00%> (ø)
include/async_mqtt/impl/client_subscribe.hpp	80.39% <100.00%> (ø)
... and 18 more