[crashtracker] add source_type to top level telemetry data

[danielsn]

          It would be nice to have this information at the top level in telemetry payload. I noticed that there is a [Origin](https://github.com/DataDog/dd-go/blob/prod/trace/apps/tracer-telemetry-intake/telemetry-payload/header.go#L87) field that could be used for this.

Originally posted by @nsavoire in #554 (comment)