[Teams] Fetching the team roster #60

Is your feature request related to a problem? Please describe.

It is not currently possible to fetch the MSTeams Roster via the SDK.:

GET /conversations/{teamId}/members/

This is very useful to be able to initiate conversations with the MSTeams users

Describe the solution you'd like
Create a new implementation "Teams" to be able to fetch this information (similarly as it is possible to fetch ActivityMembers)

Describe alternatives you've considered
Presently I implement a HTTP GET (including author token) to fetch the list of members

Additional context
https://docs.microsoft.com/en-us/microsoftteams/platform/concepts/bots/bots-context

[enhancement]