feat: add getSessionStore api

[chentsulin]

Added following API to get the session store that configured by bottender.config.js

const { getSessionStore } = require('bottender');

const sessionStore = getSessionStore();

[codecov-io]

Codecov Report

Merging #633 into release/1.2 will increase coverage by 0.42%.
The diff coverage is 65.78%.

@@               Coverage Diff               @@
##           release/1.2     #633      +/-   ##
===============================================
+ Coverage        80.66%   81.08%   +0.42%     
===============================================
  Files               99      100       +1     
  Lines             4287     4293       +6     
  Branches          1061     1077      +16     
===============================================
+ Hits              3458     3481      +23     
+ Misses             789      764      -25     
- Partials            40       48       +8

Impacted Files	Coverage Δ
packages/bottender/src/types.ts	100% <ø> (ø)	⬆️
packages/bottender/src/index.ts	100% <100%> (ø)	⬆️
...ackages/bottender/src/session/MongoSessionStore.ts	97.29% <100%> (ø)	⬆️
packages/bottender/src/server/Server.ts	10.25% <33.33%> (+1.56%)	⬆️
packages/bottender/src/initializeServer.ts	27.27% <33.33%> (+7.27%)	⬆️
packages/bottender/src/getSessionStore.ts	70% <70%> (ø)
packages/create-bottender-app/src/shared/log.ts	0% <0%> (ø)	⬆️

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