Some further clarification and potential improvements to the API specifications

[teikuran]

Problem description
While drafting the call flows (PR#122), we identified several API-level issues that appear to require clarification or correction.

The initial findings include:

1. Update WebRTC Events to enable multiple event types in a single subscription (see API Design Guide )
2. Clarification on certain properties in WebRTC Events specification (e.g., HTTPSubscriptionRequest) which seems unused

These are currently the two concrete topics, but further findings may appear as we continue our analysis. This issue is initiated to track the discussion and potential improvements to the API specification that we find while drafting the call flows (PR#122).

Alternative solution
tbd