Expose `TeamsMessage` interface to support mocking

[atc0005]

Changes

Export the TeamsMessage interface type to better support abstracting/mocking the functionality of this library by dependent projects.

NOTE

• the fields of the interface are intentionally not exported so as to help prevent future changes/improvements from breaking client code
• this change is considered a non-breaking change as the support for creating message types compliant with this interface is unchanged

References

• GH-285