refactor: Replace sessions, kernels's status_history's type map with list

[jopemachine]

Refs #412, follow-up to #480.
Related issue: #1116.

The current implementation saves only the most recent timestamp whenever status information in status_history is updated, and all previous information is deleted.

This PR prevents the loss of timestamp information by changing the data structure of sessions, kernels's status_history to List.

Previous format:

{
  "PENDING": "2024-01-01T00:00:05Z",
  "SCHEDULED": "2024-01-01T00:00:10Z",
  "PREPARING": "2024-01-01T00:00:12Z",
  "RESTARTING": "2024-01-01T00:30:00Z",
  "RUNNING": "2024-01-01T00:30:08Z",  // overwritten if restarted
}

New format:

[
  {"status": "PENDING", "timestamp": "2024-01-01T00:00:05Z"},
  {"status": "SCHEDULED", "timestamp": "2024-01-01T00:00:05Z"},
  {"status": "PREPARING", "timestamp": "2024-01-01T00:00:05Z"},
  {"status": "RUNNING", "timestamp": "2024-01-01T00:00:15Z"},  // preserved
  {"status": "RESTARTING", "timestamp": "2024-01-01T00:30:00Z"},
  {"status": "RUNNING", "timestamp": "2024-01-01T00:30:08Z"},
]

Checklist: (if applicable)

• Milestone metadata specifying the target backport version
• API server-client counterparts (e.g., manager API -> client SDK)

---

📚 Documentation preview 📚: https://sorna--1662.org.readthedocs.build/en/1662/

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📚 Documentation preview 📚: https://sorna-ko--1662.org.readthedocs.build/ko/1662/

[jopemachine]

Tested manually by writing some temporary client code.

    @api_function
    @pass_ctx_obj
    async def get_status_history(ctx: CLIContext, self):
        """
        Retrieves the status transition history of the compute session.
        """

        try:
            with Session() as session:
                fetch_func = lambda pg_offset, pg_size: session.ComputeSession.paginated_list(fields=(
                    session_fields["status_history"],
                    session_fields["status_history_log"],
                ))
                ctx.output.print_paginated_list(
                    fetch_func,
                    initial_page_offset=0,
                    page_size=20,
                )
        except Exception as e:
            ctx.output.print_error(e)
            sys.exit(ExitCode.FAILURE)

The result is as follows.

You can see status_history via graphene still use the same format, and the status_history_log uses the migrationed format.

[achimnol]

We also need to update clients using the session resource usage API handler's status_history field at:

backend.ai/src/ai/backend/manager/api/resource.py

Line 466 in e77c4a3

"status_history": row["status_history"] or {},

as this does not go through the GraphQL's backward-compatible field generation.

[jopemachine]

Switching to another PR as we introduce Graphite.
See #2116.