Add DSC attributes + trace root project onto every span

[constantinius]

For Dynamic Sampling we need to know the trace root transaction and project, so we should write the whole DSC onto the span. The trace root project is currently not part of the DSC attributes, so this would need to be extracted from another source.

[linear-code]

RELAY-239

[Dav1dde]

PR for conventions: getsentry/sentry-conventions#358