Forward install attempt IDs for remote plugins

[copyberry]

Forward install attempt IDs for remote plugins

Why

Clients need to correlate a remote plugin installation request with a specific
installation attempt.

What changed

• Add the optional installAttemptId field to PluginInstallParams.
• Forward the value as install_attempt_id in the remote plugin install POST
  body.
• Preserve the existing empty request body when the field is omitted.

Testing

• Cover protocol serialization and remote install request bodies with and
  without an attempt ID.

[github-actions]

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