Support tag:yaml.org,2002:timestamp

[embray]

I had forgotten that there is a drafted standard for a standard timestamp tag in YAML: https://yaml.org/type/timestamp.html though I do recall seeing support for it in various YAML libraries.

We already nearly have support for this in src/core/history_entry.c which contains conformant timestamps. It would make sense to make this a first-class scalar type in libasdf...either returning the value as a struct timespec or maybe a thin wrapper around it that can also hold timezone information.