wires the OpenMLS AppDataUpdate flow into the per-field metadata path

[tylerhawkes]

Wire OpenMLS AppDataUpdate proposals into the per-field metadata path

• Adds crates/xmtp_mls/src/groups/app_data/ with helpers to accumulate, encode, apply, and validate AppDataUpdate proposals against an OpenMLS AppData dictionary.
• Adds component_source.rs to centralize per-ComponentId read/encode/apply logic, bridging the new AppData dictionary and legacy group context extensions.
• Group::sync now routes metadata updates through inline AppDataUpdate commits (instead of legacy GCE updates) when proposals are enabled and a non-empty ComponentRegistry is present.
• ValidatedCommit::from_staged_commit validates inline AppDataUpdate proposals against component permissions, rejecting commits from unauthorized proposers.
• Group::mutable_metadata overlays AppData dictionary values on top of legacy extension values so callers see a unified view.
• Key packages and newly created groups now advertise AppDataUpdate proposal support in their capabilities.
• Behavioral Change: metadata update commits follow the AppData path when the registry is non-empty and proposals are enabled; groups without a registry or with proposals disabled continue to use the legacy path.

^{Macroscope summarized 586de5b.}

[tylerhawkes]

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This pull request is not mergeable via GitHub because a downstack PR is open. Once all requirements are satisfied, merge this PR as a stack on Graphite.
Learn more

• wires the OpenMLS AppDataUpdate flow into the per-field metadata path #3460 👈 (View in Graphite)
• Add component-based permission system for app data #3413
• Add TlsSet data structure backed by TlsMap<K, ()> #3410
• Update latest protos #3409
• main

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