Add `beforeSendTransaction`

[vladanpaunovic]

Problem Statement Similar to beforeSend, we will add beforeSendTransaction to allow people to strip Pii or drop transactions before they are sent to Sentry.

• [x] Python https://github.com/getsentry/sentry-python/issues/1738
• [x] Ruby https://github.com/getsentry/sentry-ruby/issues/1937
• [x] PHP https://github.com/getsentry/sentry-php/pull/1424
• [x] Javascript https://github.com/getsentry/sentry-javascript/pull/6121
• [x] Java https://github.com/getsentry/sentry-java/issues/2358
• [x] Go https://github.com/getsentry/sentry-go/issues/500
• [x] .NET https://github.com/getsentry/sentry-dotnet/issues/2077

[mattjohnsonpint]

See also: https://github.com/getsentry/team-mobile/issues/72