(opentracing-shim): Add Close operation

[srikanthccv]

If this operation is implemented for a specific OpenTracing language,
it MUST close the underlying TracerProvider if it implements a "closeable" interface or method;
otherwise it MUST be defined as a no-op operation.

The Shim layer MUST protect against errors or exceptions raised while closing the
underlying TracerProvider.

Spec: open-telemetry/opentelemetry-specification#3048

[nstawski]

I would like to take this issue.

[srikanthccv]

@nstawski mentioned there is no Close operation in OpenTracing Python Tracer. We don't need to add it in the shim package. Thanks @nstawski.