opa eval: add --timeout flag (#3824)

A command that previously just ran for a very long time can now be given
a deadline (analogously to `opa test`):

    $ opa eval -fpretty --timeout 1s 'numbers.range(1, 100000000)[_] > 10000000'
    1 error occurred: eval_cancel_error: numbers.range: timed out before generating all numbers in range

Signed-off-by: Stephan Renatus <stephan.renatus@gmail.com>