feat: add getter and method to label intent for intent tracking purpose

[chentsulin]

context.intent; // null

context.setIntent('hello-world'); 

context.intent; // 'hello-world'

This may help us build up the integration with chatbase and dashbot.

[codecov-io]

Codecov Report

Merging #617 into release/1.2 will increase coverage by 0.01%.
The diff coverage is 100%.

@@              Coverage Diff               @@
##           release/1.2    #617      +/-   ##
==============================================
+ Coverage        80.79%   80.8%   +0.01%     
==============================================
  Files               95      95              
  Lines             4270    4273       +3     
  Branches          1046    1046              
==============================================
+ Hits              3450    3453       +3     
  Misses             779     779              
  Partials            41      41

Impacted Files	Coverage Δ
packages/bottender/src/context/Context.ts	95.55% <100%> (+0.31%)	⬆️

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