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.

• Python Add beforeSendTransaction sentry-python#1738
• Ruby Add beforeSendTransaction sentry-ruby#1937
• PHP Introduce before_send_transaction option sentry-php#1424
• Javascript feat(core): Add beforeSendTransaction sentry-javascript#6121
• Java Add beforeSendTransaction sentry-java#2358
• Go Add beforeSendTransaction sentry-go#500
• .NET Support BeforeSendTransaction sentry-dotnet#2077

[mattjohnsonpint]

See also: getsentry/team-mobile#72