Improve `--message-size` argument in iggy bench: allow random message size from range.

[hubcio]

Use same format as rust range syntax.

[seroze]

@hubcio

--message-size 100..500

this would create benchmark message payloads of size between 100 to 500 randomly is that the intention ?

[hubcio]

@hubcio

--message-size 100..500

this would create benchmark message payloads of size between 100 to 500 randomly is that the intention ?

yes!