Include specversion in CloudEvent.GetPopulatedAttributes

Summary

CloudEvent.GetPopulatedAttributes() currently excludes specversion, even though the spec version is always populated on a CloudEvent. Callers that enumerate context attributes therefore omit a required CloudEvents attribute unless they special-case CloudEvent.SpecVersion.VersionId.

Cross-SDK behavior

The Java SDK includes specversion in its equivalent enumeration:

• SpecVersion.V1.getAllAttributes() includes specversion.
• CloudEventAttributes.getAttributeNames() filters that collection to non-null values.
• CloudEventImplTest.testGetAttributeNames() explicitly expects specversion.

This difference surfaced while mapping a C# CloudEvent to protocol properties by iterating GetPopulatedAttributes(); the resulting event had the other populated attributes but no specversion.

Proposed behavior

Return CloudEventsSpecVersion.SpecVersionAttribute with SpecVersion.VersionId from GetPopulatedAttributes(), alongside other populated context and extension attributes.

The built-in formatters and bindings already serialize spec version separately, so they should skip this pair during their existing enumeration to keep wire output unchanged.

I intend to submit a PR with the implementation, regression test, and updates to internal enumeration call sites.

Start at CloudEvent.GetPopulatedAttributes() and trace the formatter and binding enumeration call sites mentioned in the issue. Compare the existing C# behavior with the cited Java attribute enumeration tests, then add a regression test showing specversion is returned while serialized wire output remains unchanged.