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.

Commencez par la documentation décrivant capture_message, puisque l’issue l’identifie comme le chemin de validation actuel. Suivez la manière dont une capture est transmise et déterminez le comportement attendu de la validation locale et de la validation phone-home. Le travail est terminé lorsqu’une API peut indiquer si la configuration et la transmission ont réussi, avec un résultat exploitable pour les pages de diagnostic et les tests du Helm chart.

Rédigé par le modèle d'indexation à partir du texte de l'issue.