Emit `transaction.data` inside `contexts.trace.data`

[cleptric]

We need to ensure that transaction.data is sent as contexts.trace.data so we can also attach/extract metrics from transaction data.

## SDKs
- [x] JavaScript SDK
- [x] Electron SDK
- [x] Python SDK https://github.com/getsentry/sentry-python/issues/3372
- [x] PHP SDK
- [x] Laravel SDK
- [x] Symfony SDK
- [x] Ruby SDK https://github.com/getsentry/sentry-ruby/issues/2363
- [x] Capacitor SDK
- [x] Cordova SDK
- [ ] https://github.com/getsentry/sentry-dart/issues/2257
- [ ] https://github.com/getsentry/sentry-java/issues/3626
- [ ] https://github.com/getsentry/sentry-cocoa/issues/4248
- [x] ~Kotlin Multiplatform SDK~ (depends on native sdks)
- [x] Java SDK - covered with Android, see above
- [ ] https://github.com/getsentry/sentry-react-native/issues/4011
- [ ] https://github.com/getsentry/sentry-native/issues/1074
- [x] Elixir SDK (NA - no performance yet)
- [x] Unreal SDK - covered by the respective native SDKs (Cocoa, Android, Native)
- [x] Rust SDK https://github.com/getsentry/sentry-rust/pull/739
- [x] .NET SDK https://github.com/getsentry/sentry-dotnet/issues/3873
- [x] Unity SDK - covered by .NET, see above.
- [x] PowerShell SDK - covered by the .NET, see above.
- [x] Go SDK https://github.com/getsentry/sentry-go/issues/863

[jan-auer]

Example usage for this in Python is:

with scope.start_transaction(name="hello"):
    span_or_tx = sentry_sdk.get_current_span()
    span_or_tx.set_data("foo", "bar")

[Lms24]

I checked for JS and we don't set transactionEvent.data at all, but only transactionEvent.contexts.trace.data. Is this alright?

(Fwiw, in JS we no longer have setData or setTag APIs but only setAttribute which always populates span.data for child spans or event.contexts.trace.data for the root span (i.e. transaction event))

[jan-auer]

@Lms24 yes, that's the right way. We expect root span data (i.e. span data of the transaction) to be placed into contexts.trace.data.

There is no data field on transactions. I believe @cleptric is referring to transaction._data in the case of the Python SDK, which is the transaction's own span's data.

[cleptric]

In some SDKs, transactions are inherited from span and, therefore, have a data property. The change we need to make is to attach this data to the transaction envelope item payload under contexts.trace.data.