API: Make SpanContext.getInvalid() public

[carlosalberto]

For Propagators and other components returning invalid SpanContext instances it would be handy to share the SpanContext.getInvalid() instance, which is currently private.

If I remember correctly, there were a few reasons having keeping it private, mostly around users not abusing it. Do we want to change this?

(Observe this would be mostly an optimization. Any SpanContext can be created and users can check its getValid() method. On the other hand, TraceId and SpanId do expose their invalid instances).

See #767 for example.

[DotSpy]

Trace id and span id now exposing invalid states they have

  public static SpanId getInvalid() {
    return INVALID;
  }

for now SpanContext.getInvalid() just wrapping all public methods

  private static final SpanContext INVALID =
      create(
          TraceId.getInvalid(),
          SpanId.getInvalid(),
          TraceFlags.getDefault(),
          TraceState.getDefault());

so no harm for current state of project to make it public, otherwise TraceId.getInvalid() and SpanId.getInvalid() should be protected\private too

[jkwatson]

complete with #896