Replace `GenericWriterExporter` with own structs based on `std.Io`

[inge4pres]

Describe the current state

The logs and trace SDK exporters have a "GenericWriterExporter" generic struct that is used to build comptime types for creating StdOut and InMemory exporters.

opentelemetry-zig/src/sdk/trace/exporters/generic.zig

Lines 36 to 40 in b075448

	/// GenericWriterExporter is the generic SpanExporter that outputs spans to the given writer.
	fn GenericWriterExporter(
	comptime Writer: type,
	) type {

Describe the desired state

It would be nice if we could adapt this struct to accept a *std.Io.Writer parameter directly instead of relying on runtime checks on the generic field. If we can't, we can even remove the struct and use std.Io in the exporters directly.

I think we do it already differently for the metrics SDK, since we don't rely on the generic type anymore and both Stdout and InMemory exporters have their own implementation.

Goals:

• Refactor or remove GenericWriterExporter in logs/trace SDK exporters
• Adapt tests and examples accordingly (if needed)