feat(SessionStore): support custom session store

[chentsulin]

For example, use a user-defined string as driver:

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

module.exports = {
  session: {
    driver: 'memory2',
    stores: {
      memory2: new MemorySessionStore();
    },
  },
};

#338

[codecov-io]

Codecov Report

Merging #732 into release/1.4 will increase coverage by 0.03%.
The diff coverage is 100.00%.

@@               Coverage Diff               @@
##           release/1.4     #732      +/-   ##
===============================================
+ Coverage        81.21%   81.25%   +0.03%     
===============================================
  Files              108      108              
  Lines             4616     4619       +3     
  Branches          1162     1118      -44     
===============================================
+ Hits              3749     3753       +4     
- Misses             817      824       +7     
+ Partials            50       42       -8     

Impacted Files	Coverage Δ
packages/bottender/src/types.ts	100.00% <ø> (ø)
packages/bottender/src/getSessionStore.ts	75.00% <100.00%> (+6.03%)	⬆️
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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