sdk-trace: add SpanExportersAutoConfigure

[iRevive]

Due to modularity and lack of SPI on Native/Scala.js platforms, I came up with the following solution:

OpenTelemetrySdk
  .autoConfigured(
    _.addExporterConfigurer(OtlpSpanExporterAutoConfigure[IO]) // add the external configurer here
  )
  .use { autoConfigured =>
    val sdk = autoConfigured.sdk

    for {
      tracer <- sdk.tracerProvider.get("my-tracer")
      _ <- tracer
        .span("test", Attribute("test", "test123"))
        .use(sd => IO.println(sd.context))
    } yield ()
  }

The SDK autoconfigure will be able to:

1. Configure the telemetry resource
2. Configure the tracer provider (batch span processor, etc)
3. Configure sampler
4. Partially configure span exporter (the external exporters must be registered manually)

[iRevive]

I don't have any better idea how to communicate with the user that dependency is missing. The service will not start anyway, but we will also provide a meaningful help message.

[iRevive]

Alternatively, we can make a super module 'otel4s-sdk-autoconfigure' and automatically register the OTLP exporter there.

[armanbilge]

lack of SPI on Native/Scala.js platform

Scala Native actually got support for SPI recently, but I think it will only land in the 0.5.0 release.

• Support for Java Service Providers via linktime resolved java.util.ServiceLoader.load scala-native/scala-native#3574

There is a possibility that Scala.js may also eventually implement SPI to help support WASM.

---

But I don't think either of those things should block this PR. I will take a closer look.

[iRevive]

I will merge the PR to unblock the upcoming changes.