feat(Context): add context.setAsHandled() and context.setAsNotHandled()

[chentsulin]

Add following API for tracking handling status:

context.setAsHandled();
context.setAsHandled(false);
context.setAsNotHandled();

TODOs:

• Add tests

• Use context.setAsHandled, context.setAsNotHandled in following packages:

• @bottender/dialogflow

• @bottender/luis

• @bottender/qna-maker

• @bottender/rasa

[codecov-io]

Codecov Report

Merging #624 into release/1.2 will decrease coverage by 0.28%.
The diff coverage is 100%.

@@               Coverage Diff               @@
##           release/1.2     #624      +/-   ##
===============================================
- Coverage        80.95%   80.66%   -0.29%     
===============================================
  Files               99       99              
  Lines             4357     4287      -70     
  Branches          1061     1061              
===============================================
- Hits              3527     3458      -69     
+ Misses             790      789       -1     
  Partials            40       40

Impacted Files	Coverage Δ
packages/bottender/src/console/ConsoleContext.ts	100% <ø> (ø)	⬆️
packages/bottender/src/viber/ViberContext.ts	92.85% <ø> (+0.43%)	⬆️
...ckages/bottender/src/messenger/MessengerContext.ts	96.99% <ø> (-0.22%)	⬇️
packages/bottender/src/telegram/TelegramContext.ts	53.08% <ø> (-4.51%)	⬇️
packages/bottender/src/slack/SlackContext.ts	86.2% <ø> (-2.53%)	⬇️
packages/bottender/src/line/LineContext.ts	71.95% <ø> (-0.59%)	⬇️
packages/bottender-qna-maker/src/index.ts	94.73% <100%> (+0.98%)	⬆️
packages/bottender/src/context/Context.ts	95.83% <100%> (+0.27%)	⬆️
packages/bottender-dialogflow/src/index.ts	95% <100%> (+0.55%)	⬆️
packages/bottender-rasa/src/index.ts	89.47% <100%> (+1.23%)	⬆️
... and 1 more

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