Add beforeSendSpan Hook

[kahest]

Description

Add a beforeSendSpan hook so customers can drop individual spans. JS already has beforeSendSpan hook which uses serialized span data to be forward compatible with an only spans world, and their spans are technically read-only (because they are in otel). Related issue for sentry-cocoa: https://github.com/getsentry/sentry-cocoa/issues/4083

[markushi]

Let's use the protocol object, just like we do for the beforeSendTransaction() callback. Let's ensure we also report the dropped spans (via client reports).

[cleptric]

We can't introduce such a hook, as dropping individual spans conditionally will break extrapolation. See https://develop.sentry.dev/sdk/telemetry/spans/scrubbing-data/#scrubbing-data-with-beforesendspans and https://develop.sentry.dev/sdk/telemetry/spans/filtering/ for an eventual way forward.