feat (slack): support slack block kits and view events

[darkbtf]

1. add all event types used by Slack block kits and Slack modal
2. use view.private_metadata to store channelId for bottender's session key

[codecov-io]

Codecov Report

Merging #545 into release/1.1 will decrease coverage by 0.9%.
The diff coverage is 60%.

@@               Coverage Diff               @@
##           release/1.1     #545      +/-   ##
===============================================
- Coverage        80.82%   79.92%   -0.91%     
===============================================
  Files               95       89       -6     
  Lines             4251     4030     -221     
  Branches          1032      965      -67     
===============================================
- Hits              3436     3221     -215     
+ Misses             779      773       -6     
  Partials            36       36

Impacted Files	Coverage Δ
packages/bottender/src/slack/SlackEvent.ts	92.5% <0%> (-4.87%)	⬇️
packages/bottender/src/slack/SlackConnector.ts	92.3% <100%> (+0.15%)	⬆️
packages/bottender/src/slack/SlackContext.ts	88.73% <100%> (ø)	⬆️
packages/bottender/src/router/index.ts	88.33% <0%> (-2.21%)	⬇️
packages/bottender/src/messenger/MessengerEvent.ts	98.34% <0%> (-0.07%)	⬇️
packages/bottender/src/telegram/TelegramContext.ts	57.58% <0%> (ø)	⬆️
...es/bottender/src/session/CacheBasedSessionStore.ts	100% <0%> (ø)	⬆️
packages/bottender/src/line/LineContext.ts	72.53% <0%> (ø)	⬆️
packages/bottender/src/console/ConsoleContext.ts	100% <0%> (ø)	⬆️
... and 19 more

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[darkbtf]

We need to keep channelId in private_metadata to make session key accessible
but private_metadata is a string
It's hard to append additional fields, so I make it:

{
  original: "ORIGINAL_PRIVATE_METADATA",
  channelId: "CHANNEL_ID"
}

This workaround should be specified in the documentation of chat.views.open