Expose `TeamsMessage` interface to support mocking #73

Job	Run time
CI matrix / Assert PR branch is ahead	6s
Makefile / Assert PR branch is ahead	6s
CI matrix / Evaluate loopvar scoping changes	27s
CI matrix / Build codebase (go-ci-oldstable, false)	28s
CI matrix / Lint codebase (go-ci-oldstable, false)	34s
CI matrix / Build codebase (go-ci-stable, false)	25s
CI matrix / Lint codebase (go-ci-stable, false)	30s
CI matrix / Build codebase (go-ci-unstable, true)	28s
CI matrix / Run tests (go-ci-oldstable, false)	29s
CI matrix / Lint codebase (go-ci-unstable, true)	32s
CI matrix / Run tests (go-ci-stable, false)	38s
CI matrix / Run tests (go-ci-unstable, true)	35s
CI matrix / Optional Linting / Assert PR branch is ahead	3s
Makefile / Build codebase using Makefile quick recipe	29s
Makefile / Lint codebase using Makefile	0s
Makefile / Build codebase using Makefile all recipe	0s
CI matrix / Optional Linting / Lint codebase using optional linters (go-ci-oldstable, true)	26s
CI matrix / Optional Linting / Lint codebase using optional linters (go-ci-stable, true)	34s
CI matrix / Optional Linting / Lint codebase using optional linters (go-ci-unstable, true)	28s
	7m 18s

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