Extra field extension for Json format

[Lur1an]

This PR introduces a way for users to provide a generic field injector implementation to augment json log events.

Motivation

This solves #1531 by allowing anyone to extend the fields of the JSON records as they see fit.
This is a common problem across the rust ecosystem and often ends up with hacking or building a new formatter directly to get trace_id and span_id into logs.

Solution

pub trait ExtraFields {
    /// Write additional entries into the JSON serializer.
    fn format_extra_fields<S, N, M>(
        &self,
        ctx: &FmtContext<'_, S, N>,
        event: &Event<'_>,
        serializer: &mut M,
    ) -> serde_json::Result<()>
    where
        S: Subscriber + for<'a> LookupSpan<'a>,
        N: for<'a> FormatFields<'a> + 'static,
        M: SerializeMap<Ok = (), Error = serde_json::Error>;
}

Thanks to this it is now possible to build things like a trace_id injector:

impl ExtraFields for OtelTraceId {
    fn format_extra_fields<S, N, M>(
        &self,
        ctx: &FmtContext<'_, S, N>,
        _event: &Event<'_>,
        serializer: &mut M,
    ) -> serde_json::Result<()>
    where
        S: Subscriber + for<'a> LookupSpan<'a>,
        N: for<'a> FormatFields<'a> + 'static,
        M: SerializeMap<Ok = (), Error = serde_json::Error>,
    {
        if let Some(span) = ctx.lookup_current() {
            let extensions = span.extensions();
            if let Some(otel) = extensions.get::<OtelData>() {
                let trace_id = otel.parent_cx.span().span_context().trace_id();
                serializer.serialize_entry("trace_id", &trace_id.to_string())?;
            }
        }
        Ok(())
    }
}

The trait is kept generic and given direct access to the serializer, event and context for both efficiency and flexibility.