Add support for SVTA2053 Ad Creative Signaling in DASH EventStream

[avelad]

Summary

This issue proposes adding support for the SVTA2053 Ad Creative Signaling v2
specification in livesim2's DASH output, via the EventStream carriage mechanism.

Spec: https://www.svta.org/product/svta2053/

Background

SVTA2053 defines an interoperable signaling scheme for ad creative assets in
stitched DASH and HLS presentations. Version 2 introduces rich metadata carriage
via Pod and Slot objects, including tracking events, verification resources,
skip offsets, and click-through URIs.

The scheme URI is urn:svta:advertising-wg:ad-creative-signaling.

DASH Carriage

For each Period containing ad assets, an EventStream must be inserted with
@schemeIdUri set to the scheme URI above, and one Event per creative with its
v2 JSON payload in the node data:

{
  "version": 2,
  "type": "slot",
  "payload": [{
    "type": "linear",
    "start": 0.0,
    "duration": 30.03,
    "identifiers": [{"scheme": "urn:com:example:ads:id", "value": "abc-123"}],
    "tracking": [
      {"type": "impression", "urls": ["https://tracking.example.com/imp"]},
      {"type": "progress", "offset": 0.0, "urls": ["https://tracking.example.com/start"]}
    ]
  }]
}

Motivation

Shaka Player already supports SVTA2053 v2, so having livesim2 emit these signals
would enable end-to-end testing of ad measurement workflows without requiring a
full SSAI stack.

[avelad]

@tobbee FYI

[tobbee]

@avelad Thanks. Looks terrible to have JSON embedded in XML instead of making a new namespace, but I'll try to look into how to add this in addition to using plain DASH Ed. 6.