add support for RFC 9297 Capsule Protocol

[marten-seemann]

Fixes #486.

This turned out to be more text than I expected, but it's mostly just copy-pasted from the respective HTTP/3 section.

[Copilot]

Pull Request Overview

This PR adds support for the Capsule Protocol from RFC 9297 by defining a new http namespace for qlog events. The changes extend the existing HTTP/3 qlog event schema to include capsule-related events for logging capsule creation and parsing operations.

• Added a new http namespace with events for capsule protocol operations
• Updated abstract and introduction to reflect expanded scope beyond HTTP/3
• Defined CDDL schemas for capsule events and data types

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[Copilot]

Both references point to the same RFC 9297. The second reference should clarify that it's specifically about the Capsule Protocol within RFC 9297, or use a more descriptive reference name like 'CAPSULE-PROTOCOL' to distinguish it from the H3-DATAGRAM reference.

[Copilot]

The comment indicates 'any key-value map' but the CDDL definition restricts keys to text type only. The comment should be updated to reflect this constraint: '; The HTTPCapsule is a key-value map with text keys (e.g., JSON object)'

Suggested change

	; The HTTPCapsule is any key-value map (e.g., JSON object)
	; The HTTPCapsule is a key-value map with text keys (e.g., JSON object)

[LPardue]

We could probably make this part clearer with something like

extensions: Extended CONNECT ({{!EXTENDED-CONNECT=RFC9220}}), PRIORITY_UPDATE ({{Section 7 of !PRIORITIZATION=RFC9218}}), and HTTP/3 Datagrams {{Section 2.1 of !DGRAM-AND-CAPSULE=RFC9297}}). It also defines a schema for the HTTP Capsule Protocol ({{Section 3 of DGRAM-AND-CAPSULE}}.

[LPardue]

The design aspects of this look good to me. We just need to answer the scoping question to see whether it lives in this document or not.