add top off subcommand

it is desirable to have a subcommand that for each account checks if it has sufficient funds and if not tops off. At the same time some accounts might have too much of sol, which might be desirable to balance as well. So it might have two arguments -- how much to top off (with units as usual) and if we want to balance those accounts which have too much of sol.

Start by locating the existing CLI subcommand definitions and account-balance handling; the issue names no files or tests. Clarify the top-off amount, units, account selection, and optional excess-SOL balancing behavior, then define completion around both requested modes working safely.

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