ISO 8601 duration serializer supporting microseconds

[solwey-wes]

Added a new serializer with support for sub-second durations in ISO 8601 strings. Eg: "PT9.977S". I think this regex is solid and backwards compatible but worth a double-check...my regex game was pretty weak at the start of this task and GPT4 wasn't very helpful :P

I didn't try to add support for the longer units of time (weeks/months/years) since they weren't relevant to my use case.

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[davidmorgan]

Thanks.

I think we would only merge this if there is sufficient interest, i.e. if others are asking for it too. Otherwise it's simpler if you keep it as a local change. Thanks!