fix: Cast Text and Speak to string format

[Danieladu]

Fixes #3832

Description

In Composer, each response would create such two default templates:

# SendActivity_DidNotUnderstand()
[Activity
    Text = ${SendActivity_DidNotUnderstand_text()}
]

# SendActivity_DidNotUnderstand_text()
- ${{grandparent: {parent1: {child1: "A", child2: "B"}, parent2: {child1: "C"}}}}

So, if the value of ${SendActivity_DidNotUnderstand_text()} is json object, the text of Activity would be object too.
The error would occur while send the activity to bot service.

Specific Changes

Cast text and speak into string format while the result is not string.

Other solutions

If we could get the property type and check if the value is this type, that would be perfect. (No suitable implementation method has been found at present)
For example,

Interface Activity {
 text: string;
 speak: string;
}

something like this:

const a: unknown= xxx;
if (IsInstanceOf(a, Activity['text'])) {
   return a;
} else {
  return CastToType(a, Activity['text'])
}

@joshgummersall is that possible?

[coveralls]

Pull Request Test Coverage Report for Build 994446427

• 4 of 4 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.005%) to 84.324%

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Totals
Change from base Build 994445849:	0.005%
Covered Lines:	19633
Relevant Lines:	22087

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💛 - Coveralls