Create RFC on how to improve control over handling of sensitive data

[antonpirker]

Right now one can control what sensitive data is collected with:

• the sendDefaultPII option
• beforeSend hook
• beforeSendTransaction hook
• event processors (but only for internal use)

We need to improve this to give the users an easy and concise way to customize how their sensitive data is and what data is collected.

Lets create an RFC to specify how we want to control sensitive data.