Validate connection API

**Describe the idea**
Right now, the only way to validate that things are well-configured is to do a capture_message call (per your docs) and then go look at the Sentry UI.

It would be very helpful to have an api like `Sentry.validate_configuration` that checked the settings locally and then actually phoned-home as well to make sure that the DSN and such were good.

**Why do you think it's beneficial to most of the users**
This would let one programatically confirm all was well. It would be very helpful in a diagnostics page in the app (important thing we have and care about) but also things like Helm chart tests.

**Possible implementation**
You could just do a capture_message internally but just wrap it in something blocking that confirms the transmission actually happened successfully. That would almost be ideal as it is the "real" codepath. Then on the back-end, you could filter those messages out, or even just default them to Ignored.

Beginne mit der Dokumentation zu capture_message, da das Issue es als aktuellen Validierungspfad identifiziert. Verfolge, wie eine Capture übertragen wird, und ermittle das erwartete lokale Validierungsverhalten sowie das Validierungsverhalten beim Phone-home. Als erledigt gilt die Aufgabe, wenn eine API melden kann, ob Konfiguration und Übertragung erfolgreich waren, mit einem nutzbaren Ergebnis für Diagnose-Seiten und Helm chart-Tests.

Vom Indexierungsmodell aus dem Issue-Text verfasst.