Add initial file structure for FrontlineSms

Summary:
Copy same structure as SMSSync plugin since they have similar
implementation

* Add FrontlineSms as a data provider
* Make request GET only

Test Plan:
Run the following command

  ./bin/behat --config application/tests/behat.yml application/tests/features/data-provider/frontlinesms.feature --format progress

Reviewers: shadowhand, rjmackay

Reviewed By: rjmackay

Subscribers: lkamau

Maniphest Tasks: T93

Differential Revision: https://phabricator.ushahidi.com/D32