want time range options for `omdb db migration ls`

Follow-up from #6160.

Currently, the omdb db migration list command will show the n most recent migrations matching the filter, where n is the provided --fetch-limit argument. This is fine for viewing the most recent matching migrations, but makes it hard to inspect migrations that happened in the past without listing everything. It would be nice to add the option to select a time window for listing migrations.

This all seems good as far as using the new index. Does it make sense to let the caller / CLI provide timestamps themselves? E.g., they could look at migrations in the last hour, if they wanted, or the code could use the UNIX epoch if nothing was provided.

Part of why I ask is we have given control of the limit in the CLI, but not the starting point.

Originally posted by @bnaecker in https://github.com/oxidecomputer/omicron/pull/6160#discussion_r1695999726