[config] Make TraceNotification configurable

[EricMountain]

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Makes TraceNotification configurable.

When enabled, trace notifications (send_trace_notify()) consume CPU in proportion to the volume of traffic on an instance. This makes the use of Hubble or other perf event consumers such as cilium monitor prohibitive on high-traffic nodes.

TraceNotification is only configurable via the agent API (e.g. cilium config TraceNotification=Disabled) and defaults to Enabled. This change makes this setting configurable at agent startup.

Fixes: #issue-number

TraceNotification is now configurable via `--trace-notification=true|false`.

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[EricMountain]

Superseded by #30063