feat(Server): support custom connectors

[chentsulin]

• Support custom connectors in bottender.config.js.
• Add some tests for Server.ts.

[codecov-commenter]

Codecov Report

Merging #781 into release/1.5 will increase coverage by 0.93%. The diff coverage is 85.71%.

@@               Coverage Diff               @@
##           release/1.5     #781      +/-   ##
===============================================
+ Coverage        81.08%   82.01%   +0.93%     
===============================================
  Files              108      108              
  Lines             4747     4754       +7     
  Branches          1197     1153      -44     
===============================================
+ Hits              3849     3899      +50     
+ Misses             862      825      -37     
+ Partials            36       30       -6     

Impacted Files	Coverage Δ
packages/bottender/src/types.ts	100.00% <ø> (ø)
packages/bottender/src/server/Server.ts	63.44% <85.71%> (+52.97%)	:arrow_up:
packages/bottender/src/chain.ts	76.92% <0.00%> (ø)
packages/bottender/src/getClient.ts	55.55% <0.00%> (ø)
packages/bottender/src/router/index.ts	90.66% <0.00%> (ø)
packages/create-bottender-app/src/shared/log.ts	0.00% <0.00%> (ø)

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