api: storage_service/snapshots: support skip-flush option
api: storage_service/snapshots: support skip-flush option

The option is provided by nodetool snapshot
https://docs.scylladb.com/operating-scylla/nodetool-commands/snapshot/
```
nodetool [(-h <host> | --host <host>)] [(-p <port> | --port <port>)]
     [(-pp | --print-port)] [(-pw <password> | --password <password>)]
     [(-pwf <passwordFilePath> | --password-file <passwordFilePath>)]
     [(-u <username> | --username <username>)] snapshot
     [(-cf <table> | --column-family <table> | --table <table>)]
     [(-kc <kclist> | --kc.list <kclist>)]
     [(-sf | --skip-flush)] [(-t <tag> | --tag <tag>)] [--] [<keyspaces...>]

-sf / –skip-flush    Do not flush memtables before snapshotting (snapshot will not contain unflushed data)
```

But is currently ignored by scylla-jmx (scylladb/scylla-jmx#167)
and not supported at the api level.

This patch wires the skip_flush option support to the
REST API.

Fixes #8725

Test: unit(dev)
Signed-off-by: Benny Halevy <23c997c90eb8537635e9392d48e7af1bf1b2ca22@scylladb.com>

scylladb/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla