Support SpanTraces

I was reading https://greptime.com/blogs/2024-05-07-error-rust just now via https://news.ycombinator.com/item?id=42457515 and wondered what it would take to support [SpanTraces](https://docs.rs/tracing-error/latest/tracing_error/struct.SpanTrace.html) from the [tracing-error crate](https://docs.rs/tracing-error/latest/tracing_error/) directly in derived Error implementations. It turned out to be simpler than I expected (30 mins work to implement).

This is an example of what this makes possible:

```rust
fn main() {
tracing_subscriber::registry()
.with(tracing_subscriber::fmt::layer())
.with(tracing_error::ErrorLayer::default())
.init();

let result = boom();
match result {
Err(err) => {
eprintln!("error: {}", err);
match err {
Error::MyError(source, span_trace) => {
eprintln!("source: {}", source);
eprintln!("span trace: {:#?}", span_trace);
}
}
}
_ => unreachable!(),
}
}

#[tracing::instrument]
fn boom() -> Result<(), Error> {
inner_boom()?;
Ok(())
}

#[tracing::instrument]
fn inner_boom() -> Result<(), Error> {
non_span_trace()?;
Ok(())
}

#[tracing::instrument]
fn non_span_trace() -> std::io::Result<()> {
std::fs::read_to_string("nonexistent-file")?;
Ok(())
}

#[derive(Error, Debug)]
enum Error {
#[error("I/O error: {0}")]
MyError(#[from] std::io::Error, SpanTrace),
}
```

Produces:

```
error: I/O error: No such file or directory (os error 2)
source: No such file or directory (os error 2)
span trace: SpanTrace [
{ target: "spantrace", name: "inner_boom", file: "examples/spantrace.rs", line: 32 },
{ target: "spantrace", name: "boom", file: "examples/spantrace.rs", line: 26 },
]
```

This is instead of changing all the various places where traced errors are needed to return TracedErrror and manually calling `some_fallible_method().in_current_span()?` everywhere.

PR #401 implements this, but I figured it was probably nicer to discuss the idea in an issue rather than in a PR.

The obvious downside to the implementation is that `tracing-*` is not `1.x` yet, so realistically the code would not be able to be merged as yet (except perhaps behind an unstable feature flag / compiler flag).

Would love to hear some thoughts on this, particularly if they can suggest a direction that would make this work properly (perhaps a thiserror-tracing fork which closely tracks thiserror releases would work?)