[sdk-trace] make resource private in BasicTracerProvider

[pichlermarc]

Description

resource is currently public and incorrectly allows users to modify resource attributes by setting (provider.resource.attributes["some"] = "value") on the fly even though the resource should be immutable.

This can have unintended side-effects on the exported telemetry and should therefore be avoided.

This issue is considered done when

• the resource property at

  opentelemetry-js/packages/opentelemetry-sdk-trace-base/src/BasicTracerProvider.ts

  Line 74 in 47444f2

  readonly resource: IResource;

  has been marked private or protected
• it has been verified that resource is not public in any other way for the BasicTracerProvider
• it has been verified that resource is not public in any other way for the WebTracerProvider
• it has been verified that resource is not public in any other way for the NodeTracerProvider