Add `BeforeSendCheckIn` callback Option

[Flash0ver]

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 getsentry/sentry-docs#14972

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

[linear]

NET-455 Add `BeforeSendCheckIn` callback Option