feat: add request metadata to the conversation history and emitted telemetry

[brandonskiser]

Description of changes:
Adding metadata about our send message requests, storing it as part of the conversation history, and sending it as part of ChatAddedMessage telemetry.

Details

Couple things to call out:

1. ChatAddedMessage telemetry is now correctly being sent in all error cases, and sent after tool use execution. We may need to add more timing information after tool use execution completes, but this is omitted for now.
2. A new type RequestMetadata is created for including information about how the response stream is handled (contains timing info between chunks, total size, etc.). This is stored in the conversation history.
3. To prevent even more code changes, a crazy hack for getting RequestMetadata on sigint is seemingly required by the use of an Arc<Mutex<Option<RequestMetadata>>> value. This is essentially shared state between the response handler and the sigint handler, and is required because the sigint handler has no notion of what the stream output would be otherwise. This isn't ideal, but seems to be required with the current code setup without a larger refactor around ctrl+c handling.
4. Adding a new telemetry type codewhispererterminal_recordUserTurnCompletion to the definitions, but is currently unimplemented in the code. This will be done in a followup PR.

Important Callouts

This will break the conversation serialization format with the current version since the history is being updated from (UserMessage, AssistantMessage) to a struct HistoryEntry { user, assistant, request_metadata }.

If we want to preserve backwards compatibility, then a separate PR could be made adding versioning to the conversation state instead, but doing that as part of this PR would be too much work.

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