fix(webhook): typing webhook request handler

[yhsiang]

1. typing type of webhook event which is not found in the '@line/bot-sdk'
2. typing context

[yhsiang]

got this when running eslint:fix

 193:5   error    'input' is never reassigned. Use 'const' instead    

[coveralls]

Pull Request Test Coverage Report for Build 5863817242

• 8 of 14 (57.14%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.03%) to 91.049%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/webhook/index.ts	8	14	57.14%

Totals
Change from base Build 5392086039:	-0.03%
Covered Lines:	978
Relevant Lines:	1050

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💛 - Coveralls

[MrOrz]

As discussed on Slack we decided to create a new ticket for typing handlers/tutorial and removing ts-ignore here.

[MrOrz]

Seems that the root cause of many otherField as OOO is that we separate type from other fields.

We could have used discriminated union to properly narrow down the message type, but we somehow removed the discriminator (type) from the otherField, making narrowing impossible. We can merge this first and see if we can remove this otherField together in the future.