Ability to further limit session duration

[tammersaleh]

While the server limits how long a session token lasts via server.targets.TARGET.aws.session_duration, the end user should be able to specify an override duration. The shorter of the two durations should be used.

Can specify by command line flag:

$ cludo exec --duration=10m -- command

...or ENV var:

$ export CLUDO_DURATION=10m
$ cludo exec -- command

Should support common time units (h, m, s)