in this series, we encode the value of generation using UUID to prepare for the UUID generation identifier. simpler this way, as we don't need to have two ways to encode integer or a timeduuid: uuid with a zero timestamp, and a variant. also, add a `from_string()` factory method to convert string to generation to hide the the underlying type of value from generation_type's users.
Closes#13782
* github.com:scylladb/scylladb:
sstable: use generation_type::from_string() to convert from string
sstable: encode int using UUID in generation_type
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