Add ability to query and export spans to dataset (#574)

This PR adds two new methods to the telemetry API:
1) Gives the ability to query spans directly instead of first querying
traces and then using that to get spans
2) Another method save_spans_to_dataset, which builds on the query spans
to save it on dataset.

This give the ability to saves spans that are part of an agent session
to a dataset.

The unique aspect of this API is that we dont require each provider of
telemetry to implement this method. Hence, its implemented in the
protocol class itself. This required the protocol check to be slightly
modified.

llama_stack/providers/registry/telemetry.py

@@ -18,6 +18,7 @@ def available_providers() -> List[ProviderSpec]:
                 "opentelemetry-sdk",
                 "opentelemetry-exporter-otlp-proto-http",
             ],
+            api_dependencies=[Api.datasetio],
             module="llama_stack.providers.inline.telemetry.meta_reference",
             config_class="llama_stack.providers.inline.telemetry.meta_reference.config.TelemetryConfig",
         ),