User segment should be a top level property on the user

[adinauer]

Problem Statement

Currently the user_segment is being read from other['segment'].

Solution Brainstorm

A top level property called segment on the user should be used as is done in the JS SDK:

https://github.com/getsentry/sentry-javascript/blob/beb3c973c0f10c538c8369983fc08c5bd031bfde/packages/types/src/user.ts#L8

[AbhiPrasad]

Develop docs amended here: https://github.com/getsentry/develop/pull/685