Add Span.set_kind(SpanKind) or Span.update_kind(SpanKind) methods to API

[owais]

This can be useful if a child span needs to influence parent span's kind. For example, if multiple client libraries that wrap each other are all instrumented. For example: open-telemetry/opentelemetry-python-contrib#456

[mariojonke]

There might be exporters which already export a span on start so updating the span kind can be problematic in this case.

[codeboten]

This should be reflected in the spec if we decide to move ahead with it.

[srikanthccv]

There hasn't been any interest in this. I think we should first get this added in spec. We are closing this for now. Re-open and pursue at the spec level if people are still interested. Thanks.