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

• Ran make prepush.

[bradjc]

I think we want to do something like this, but I get the sense that what is in debug.rs right now is more based on what worked rather than something meant to be extensible.

I think we should create the trait, but having extract() as a member function doesn't make much sense. Probably flush() should be there instead.

The existing struct should be called DebugWriterRingBuffer or something.

[bradjc]

Suggested change

	pub trait DebugWriterTrait: Write + IoWrite {
	pub trait DebugWriter: Write + IoWrite {

[ZhekaS]

Yes, the draft implementation is a somewhat dirty WA intended to keep all the existing implementation as is while also allowing custom one. Ideally the empty trait functions should not be there as they are only required by specific implementations. If we are in agreement we want this, I can look into cleaner implementation when I have some free cycles...