Add trace metric protocol models and envelope item container support

[szokeasaurusrex]

This issue captures the protocol/envelope portion of #997, so later client plumbing can build on a small, reviewable base.

Scope from #997

• sentry-types/src/protocol/v7.rs
• sentry-types/src/protocol/envelope.rs

Details

Extract only the trace metric model and envelope container implementation described in the trace metrics spec: metric type enum, metric payload fields, trace_metric envelope item type, and encode/decode handling including correct content type and item count behavior. Keep this part free of client capture or macro API concerns, and include round-trip coverage in sentry-types to show metric items survive serialization/deserialization intact.

[linear]

RUST-159