Add client_address into standalone spans #15643
Description
Problem Statement
In getsentry/relay#4047 we started extracting user information from standalone spans. However, in order for this to fully work, client_address must be sent in the data section of a span from the SDKs.
In the end, all this is to support the subregion selector in the web vitals module getsentry/sentry#75230
Solution Brainstorm
Add a new property to span.data, client_address. This is set to either the client itself, or {{auto}} if we want relay to derive it. We already do something similar under the the user section of a transaction