[DISCO-3243] Upgrade to UniFFI 0.29.0

[bendk]

This mostly consisted of upgrading the custom types code and adding [Remote] attributes. All-in-all, not too bad and I prefer the new syntax.

Tasks before merging:

• Glean release with the UniFFI upgrade
• moz-central patch that upgrades uniffi-bindgen-gecko-js

Pull Request checklist

• Breaking changes: This PR follows our breaking change policy
  • This PR follows the breaking change policy:
    • This PR has no breaking API changes, or
    • There are corresponding PRs for our consumer applications that resolve the breaking changes and have been approved
• Quality: This PR builds and tests run cleanly
  • Note:
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• Tests: This PR includes thorough tests or an explanation of why it does not
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Branch builds: add [firefox-android: branch-name] to the PR title.