spec: sampler should receive Context instead of SpanContext

[dyladan]

https://github.com/open-telemetry/opentelemetry-specification/blob/06d16d7db88c3e266636f84887de61e3363b5429/specification/trace/sdk.md#shouldsample

Required arguments:

• Context with parent Span. The Span's SpanContext may be invalid to indicate a root span.
• TraceId of the Span to be created. If the parent SpanContext contains a valid TraceId, they MUST always match.
• Name of the Span to be created.
• SpanKind of the Span to be created.
• Initial set of Attributes of the Span to be created.
• Collection of links that will be associated with the Span to be created. Typically useful for batch operations, see Links Between Spans.