Add client_address into standalone spans

[DominikB2014]

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

[AbhiPrasad]

Hmm @cleptric how do we want to approach the auto detection stuff for ips and similar in standalone spans considering what we've run into for transactions/errors?

[cleptric]

I'll get back to you!