Add Connect subscriptions [ch4619]

[hassansin]

Added ConnectSubscription method: https://dev.chartmogul.com/reference#connect-subscriptions

example:

err := api.ConnectSubscriptions("cus_f466e33d-ff2b-4a11-8f85-417eb02157a7", []cm.Subscription{
	{
		ExternalID:     "d1c0c885-add0-48db-8fa9-0bdf5017d6b0",
		DataSourceUUID: "ds_ade45e52-47a4-231a-1ed2-eb6b9e541213",
	},
	{
		ExternalID:     "9db5f4a1-1695-44c0-8bd4-de7ce4d0f1d4",
		DataSourceUUID: "ds_ade45e52-47a4-231a-1ed2-eb6b9e541213",
	},
})

[pkopac]

Should we set any expectations in the test after the connect subscriptions is called? I mean, what actually is the effect? I know we're supposed to test just the client library here, but just calling it and expecting no error seems to me kind of "unfinished".

[hassansin]

@pkopac good point, I'm waiting for Vinay to provide me with more details about verifying the effect through API.