Allow custom implementations of DebugWriter

[ZhekaS]

Pull Request Overview

Fixes #4302

Add a trait for debug writer, so the downstream code can implement it's own, not depending on UART backend.

Testing Strategy

Tested on QEMU and local simulated RiscV platform with both UART based and non-UART implentations. Tested debug buffer overflow on buffered implementation

TODO or Help Wanted

Would be nice to test with some implementation using console_ordered as it has some "intrusive" calls to the debug api

Documentation Updated

Inline documentation added in debug.rs

Formatting

• [x] Ran make prepush.