Enforce size limits in feedback envelope processors

[aliu39]

The long-term solution / Follow-up for:

• fix(feedback): enforce a max message size from all sources #79326
• UF Backend: Limit feedback message size #76298

Some context: #79326 (comment)

• Check for message size limits on the envelope schemas, in rust. Add them if they don't exist. Make sure they're <= 4096 chars as specified in the PR above. (For envelopes we prefer to use bytes.)

  • new feedback (user_report_v2)
  • legacy feedback (user_report)

• Rather than truncate, emit a rejected outcome for Data Category UserReportV2, and return an error for the SDK.

  • relates to https://github.com/getsentry/team-replay/issues/477

• Clean up the feedback max-size option and truncate logic in the previous PR.

A separate size limit could be enforced for spam detection, for COGS. But I wouldn't about it for this issue.

[aliu39]

This can probably be moved to the relay repo

[aliu39]

Moved to relay getsentry/relay#4379