Add SL-HDR detection support

[dbPhilips]

Hello, this issue is about adding support for detection of SL-HDR capability.

Would like to add support for detection of SL-HDR HDR dynamic metadata (specified in ETSI TS 103 433), to the HdrMetadataType structure . This had already been suggested in issue #118 (in particular this posting when this structure was still called “MetadataDescriptor” or “HDRCapability”).

Hence, propose to extend the HdrMetadataType with "etsi-ts103433" to allow user agents to detect whether it is supported. Most likely this would entail the following changes:

1. In paragraph 2.1.5 “etsi-ts103433” would have to be added to the enum;
2. Directly below it a bullet would have to be added:
   • etsi-ts103433, representing the dynamic metadata type defined by [ETSI-TS103-433].
3. In the list of normative references in section 5 the following entry would have to be added:
   [ETSI-TS-103433] High-Performance Single Layer High Dynamic Range (HDR) System for use in Consumer Electronics devices. 2021. URL: https://www.etsi.org/deliver/etsi_ts/103400_103499/10343301/01.04.01_60/ts_10343301v010401p.pdf, https://www.etsi.org/deliver/etsi_ts/103400_103499/10343302/01.03.01_60/ts_10343302v010301p.pdf, https://www.etsi.org/deliver/etsi_ts/103400_103499/10343303/01.02.01_60/ts_10343303v010201p.pdf

Can you please provide guidance on the procedure for this extension?