expose addSpanProcessor and getActiveSpanProcessor in tracer_provider

[longlho]

What does this PR do?

Expose 2 new methods in typings: addSpanProcessor and getActiveSpanProcessor for TraceProvider.

Motivation

I use addSpanProcessor primarily to chain debugging processor like

tracerProvider.addSpanProcessor({
  forceFlush: async () => {},
  onStart: (_span: Span, _parentContext: Context) => {},
  onEnd: (span: ReadableSpan) => {
    const sampled = !!(span.spanContext().traceFlags & TraceFlags.SAMPLED);
    console.log(`Span name: ${span.name}, sampled: ${sampled}`, 'span', span);
  },
  shutdown: async () => {}
});

Same for getActiveSpanProcessor.

• Unit tests.
• TypeScript definitions.
• TypeScript tests.
• API documentation.
• CircleCI jobs/workflows.
• Plugin is exported.

Additional Notes