SCTE-35 splice information signalling in EventType

[Vltnrch]

What

Add Signal and SpliceInfoSection fields to EventType, with full type coverage for ANSI/SCTE 35 2023r2 splice information sections (XSD scte_35_20230713.xsd).

Why

DASH events carrying SCTE-35 signalling appear under two bindings in the wild and both are now first-class:

• <Signal> wrapper specified by SCTE 214-1 — Unified Streaming, Broadpeak, …
• <SpliceInfoSection> directly under <Event> — AWS MediaTailor and others, valid because SpliceInfoSection is a global element in the SCTE-35 XSD
  (*EventType).SpliceInfo() returns whichever the source manifest populated.

Coverage

• Splice commands: SpliceNull, SpliceSchedule, SpliceInsert, TimeSignal, BandwidthReservation, PrivateCommand
• Splice descriptors: Avail, DTMF, Segmentation, Time, Audio, Private
• Binary, Ext and anyAttribute extensibility
• Exported constants: SCTE35Namespace, SCTE35Namespace2016, SCTE35SchemeIdXML/XMLBin/Bin, and the Table 23 segmentation type IDs

XML namespace round-trip

Two URIs are seen in the wild for the same schema:

• http://www.scte.org/schemas/35 — canonical, declared targetNamespace since 2020
• http://www.scte.org/schemas/35/2016 — legacy, still produced by a large fraction of packagers
  Each SCTE-35 type embeds an unnamed XMLName xml.Name: the URI seen on unmarshal is replayed verbatim on marshal, so a round trip preserves whichever the input used. Deliberate exception to the "output names are fixed" rule listed in the README, called out there explicitly. New* constructors default XMLName to the canonical URI for programmatic use.