Add callback for packet send function to dump the output packets.

[dmitry1945]

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Change description

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SystemView currently sends trace data only via RTT. This callback allows firmware to intercept raw packets—for example, to dump them to an alternate channel, integrate with QEMU, or debug without changing the main RTT output path.

Curretn PR adds an optional callback to intercept outgoing SEGGER SystemView packets before they are written to the RTT buffer. New API SEGGER_SYSVIEW_SetSendPacketCallback() lets you subscribe to packet transmission.
The callback is invoked while the packet is being formed, with access to the packet start/end, event ID, and timestamp.

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