[usability] Allow prefix matching for yb-admin commands

Jira Link: DB-2024 Currently when yb-admin command is slightly misspelled, e.g. list_snapshot_schedule instead of list_snapshot_schedules, the user would be given the whole list of commands.

User experience can be improved by using the (partial) command as prefix.

If there is only one match, that match would be picked - we can ask the user to confirm the selection (or add option, say -accept-prefix-match which would accept the selection without asking). In case there are multiple candidates for the given prefix, the list of commands would be printed.