Add `BeforeSendCheckIn` callback Option

### Description

### Summary

We currently do not expose a `BeforeSendCheckIn` callback option. SDKs may *optionally* offer such a callback option for `SentryCheckIn` via `CaptureCheckIn`.

To gauge impact, please upvote if this option/callback would be useful for your scenarios.

### Remarks

The docs were updated to clarify that `BeforeSend` (for Events only) should not be used for Check-Ins (and other non-Events, such as Transactions and Logs).
Check-Ins may use an *optional* `BeforeSendCheckIn`.
See [https://github.com/getsentry/sentry-docs/pull/14972]()

Question:
Do we want to add a `BeforeSendCheckIn` callback option?