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.

issue では現在の検証経路として capture_message が特定されているため、まず capture_message を説明しているドキュメントから始めます。capture がどのように送信されるかを追跡し、ローカル検証と phone-home 検証で期待される動作を明らかにします。設定と送信が成功したかどうかを API が報告でき、診断ページと Helm chart のテストで利用できる結果が得られれば完了です。

索引モデルが issue の本文から書いたものです。