this is the first step to the uuid-based generation identifier. the goal is to encapsulate the generation related logic in generator, so its consumers do not have to understand the difference between the int64_t based generation and UUID v1 based generation.
this commit should not change the behavior of existing scylla. it just allows us to derive from `generation_generator` so we can have another generator which generates UUID based generation identifier.
Closes#13073
* github.com:scylladb/scylladb:
replica, test: create generation id using generator
sstables: add generation_generator
test: sstables: use generate_n for generating ids for testing
1750bb34b7