Implement support for async activities export

[tommycarney]

This PR provides support for a new async activities export with optional params for start date, end date and activity type.

	activities_export, err := api.MetricsCreateActivitiesExport(&NewMetricsActivitiesExport{
		StartDate: "2020-01-01",
		EndDate:   "2020-12-31",
		Type:      "contraction",
	})

Initially, the export will have the status pending and a nil file_url. After a period of time, the async job will complete and the status will update to processing and then succeeded or failed. In the case of succeeded, the file_url will be populated with the url with the zipped activities export.

A user of the client library could schedule a job with the ID of the export, so they can periodically check it until it reaches succeeded or failed.

If you have the uuid of the export, you can retrieve it via:

var activities_export_id = "7f554dba-4a41-4cb2-9790-2045e4c3a5b1"
activities_export, err := api.MetricsRetrieveActivitiesExport(activities_export_id)

[shortcut-integration]

This pull request has been linked to Clubhouse Story #32313: Go async export.

[hassansin]

It's better to avoid struct name starting with New. You can use CreateMetricsActivitiesExportParam.

[hassansin]

don't use underscores in Go names; var activities_export should be activitiesExport

[hassansin]

Your code does not pass gofmt in 1 place. Go fmt your code!

May be some whitespace issue. Try formatting with gofmt.

[tommycarney]

yup, done!

[hassansin]

Suggested change

	var activitiesExportId = "7f554dba-4a41-4cb2-9790-2045e4c3a5b1"
	var activitiesExportID = "7f554dba-4a41-4cb2-9790-2045e4c3a5b1"