Add Resource type to Tracing API

OpenTelemetry defines a `Resource` as part of its spec: https://github.com/open-telemetry/opentelemetry-specification/blob/v0.7.0/specification/resource/sdk.md

> Resource captures information about the entity for which telemetry is recorded. For example, metrics exposed by a Kubernetes container can be linked to a resource that specifies the cluster, namespace, pod, and container name.

A resource is passed along when sending spans to the Otel collector.

Resources also have attributes with semantic conventions similar to spans: https://github.com/open-telemetry/opentelemetry-specification/blob/v0.7.0/specification/resource/semantic_conventions/README.md

It might be a good idea to expose the resource API as well as part of the `Tracing` library, but as the resources (e.g. which cloud provider the app is running on or whether it's running in a Docker container) are usually unknown to frameworks and instead known by the application developer, this API could also live in a specific tracer implementation.

Start by reading the linked OpenTelemetry Resource specification and semantic conventions, then inspect the existing Tracing API and tracer implementations. Resolve whether Resource belongs in the shared Tracing library or a specific tracer implementation, and define how it is passed with spans to the Otel collector. Done means the chosen API supports application-provided resource attributes and propagation is covered.

Written by the indexing model from the issue text.