add test case for coverage

[zhengjian526]

#119

1. add coverage filter for file "string_view.hpp" and "nonstd_any.hpp";
2. add test case for client async callback and publish and subscribe by token;
3. add server error callback

[github-actions]

Code Coverage Report
for detail, goto summary download Artifacts

Filename                    Functions  Missed Functions  Executed       Lines      Missed Lines     Cover    Branches   Missed Branches     Cover
-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
client_util.hpp                     3                 1    66.67%          19                 8    57.89%           2                 1    50.00%
codec.h                             4                 0   100.00%          24                 2    91.67%           0                 0         -
connection.h                       39                 6    84.62%         375                91    75.73%          72                36    50.00%
io_service_pool.h                   5                 0   100.00%          32                 1    96.88%          12                 1    91.67%
md5.hpp                            19                 2    89.47%         127                 8    93.70%          24                 0   100.00%
nonstd_any.hpp                     20                 5    75.00%          49                12    75.51%           8                 4    50.00%
router.h                           19                 2    89.47%         144                35    75.69%           4                 3    25.00%
rpc_client.hpp                     73                12    83.56%         663               143    78.43%         132                58    56.06%
rpc_server.h                       26                 3    88.46%         191                20    89.53%          40                 9    77.50%
string_view.hpp                    23                10    56.52%          72                35    51.39%          22                14    36.36%
-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
TOTAL                             231                41    82.25%        1696               355    79.07%         316               126    60.13%

[codecov-commenter]

Codecov Report

Attention: 20 lines in your changes are missing coverage. Please review.

Comparison is base (07d5735) 53.32% compared to head (ab27c06) 54.30%.

Files	Patch %	Lines
tests/test_rest_rpc.cpp	83.92%	18 Missing ⚠️
include/rest_rpc/rpc_server.h	84.61%	2 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #130      +/-   ##
==========================================
+ Coverage   53.32%   54.30%   +0.97%     
==========================================
  Files         170      170              
  Lines        7908     8038     +130     
==========================================
+ Hits         4217     4365     +148     
+ Misses       3691     3673      -18     

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[qicosmos]

LGTM