add metadata storage support in seg files

https://github.com/erigontech/erigon/pull/17090#issuecomment-3317987755

- seg to support for v0 and v1; v1 has some typed metadata (compression; start/end num; number of added elements)
- tooling to add migration routine...make it work with erigon start/downloader
- skipping "automatic snapshot reset" migration strategy.
- think about "dev UX" i.e. what code changes are needed to get a seg file (say history .v) to start using v1
- aim to have 1 domain's history (.v) files on v1 for next release? Maybe rcache...