Update license header to standard format for source files missed prior.
Add license header to new source files.
Add Makefile check to test all `*.go` and `*.sh` files have a copyright
notice (or comment about them being auto-generated) within the first few
lines.
* Refactor SDK Sampler API to conform to Spec
* Sampler is now an interface rather than a function type
* SamplingParameters include the span Kind, Attributes, and Links
* SamplingResult includes a SamplingDecision with three possible values, as well as Attributes
* Add attributes retruned from a Sampler to the span
* Add SpanKind, Attributes, and Links to API Sampler.ShouldSample() parameters
* Drop "Get" from sdk Sampler.GetDescription to match api Sampler
* Make spanID parameter in API Sampler interface a core.SpanID
* Fix types and printf format per PR feedback from krnowak
* Ensure unit test error messages reflect new reality
Co-authored-by: Joshua MacDonald <jmacd@users.noreply.github.com>
