feat(core): transaction.set_data sets data on TraceContext

[lcian]

The Rust SDK currently sets data on transaction.extra when calling transaction.set_data on a Transaction.
This change makes it so that data is instead set on the TraceContext i.e. transaction.context.trace.data as other SDKs do (see getsentry/team-sdks#95).

Required for #733

[lcian]

Changed existing APIs that access/mutate Transaction data (used in the tracing integration) to change data attributes (TraceContext.data) instead of transaction.extra.
Changed comments to reflect the new naming convention of "data attributes" to refer to TraceContext.data.
Changed tests as well to match the new API.

[Swatinem]

lgtm! its good to have someone who actually knows how all of this is supposed to work within SDKs :-)