Unionize TaskModuleResponse.task type definition

[EricDahlvang]

Fixes #1588

[coveralls]

Pull Request Test Coverage Report for Build 141977

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.07%) to 80.961%

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Totals
Change from base Build 141906:	0.07%
Covered Lines:	12915
Relevant Lines:	15236

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💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 142928

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 81.033%

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Totals
Change from base Build 142911:	0.0%
Covered Lines:	12985
Relevant Lines:	15307

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💛 - Coveralls

[EricDahlvang]

Hold merging this... @stevengum pointed out another place we should use this same union

[stevengum]

@EricDahlvang and I still want to take this PR with the additional changes he mentioned.

Pedantically speaking, this is PR is a breaking change. However, Teams will not respect TaskModuleResponseBases that aren't one of the subtypes: TaskModuleContinueResponse | TaskModuleMessageResponse, so sending anything else will result in an error.