Enforce size limits in feedback envelope processors

[aliu39]

Copied from sentry repo, since the main work to be done is in Relay.

The long-term solution / Follow-up for:

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

Some context: getsentry/sentry#79326 (comment).

TLDR; right now we're truncating feedbacks in Sentry backend if they're too large. The better solution is to drop large feedbacks immediately, in Relay.

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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.

Follow-ups:
A separate, smaller size limit could be enforced for spam detection, to save costs. But I wouldn't about it for this issue.