This implements specification requirement:
https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/resource/sdk.md#resource-creation
- Changes `resource.NewWithAttributes` to require a schema URL. The old function
is still available as `resource.NewSchemaless`. This is a breaking change.
We want to encourage using schema URL and make it a conscious choice to have a
resource without schema.
- Merge schema URLs acccording to the spec in resource.Merge.
- Several builtin resource detectors now correctly populate the schema URL.
Co-authored-by: Tyler Yahn <MrAlias@users.noreply.github.com>