feat: implement getClient helper function

[chentsulin]

Add this helper to access underlying messaging client configured by bottender.config.js:

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

const messenger = getClient('messenger');

messenger.sendText(USER_ID, 'Hello!', { tag: 'CONFIRMED_EVENT_UPDATE' });

const line = getClient('line');

line.pushText(USER_ID, 'Hello!');

[codecov-io]

Codecov Report

Merging #634 into release/1.2 will decrease coverage by 0.11%.
The diff coverage is 57.89%.

@@               Coverage Diff               @@
##           release/1.2     #634      +/-   ##
===============================================
- Coverage        81.08%   80.97%   -0.12%     
===============================================
  Files              100      101       +1     
  Lines             4293     4310      +17     
  Branches          1077     1079       +2     
===============================================
+ Hits              3481     3490       +9     
- Misses             764      771       +7     
- Partials            48       49       +1

Impacted Files	Coverage Δ
packages/bottender/src/getSessionStore.ts	68.96% <100%> (-1.04%)	⬇️
packages/bottender/src/index.ts	100% <100%> (ø)	⬆️
packages/bottender/src/getClient.ts	52.94% <52.94%> (ø)

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