cache-control CLI tweaks

[adorton-adobe]

CLI options for cache-control need some tweaks -

• --clear is misidentified as an option in the usage text when it should be in the "flags" section. I believe this is because in the Opts struct it is defined as Option<bool>. Making it a plain bool with a default of false should resolve the issue. We won't run into the same issues we did with --ssl because there is nothing in the config that this option overrides.
• We should add an option to specify the path of the DB file to maintain consistency with the update I proposed in Command-line option to define cache DB path #5.
• We should add docstrings to --export-file and --import-file noting that these options are not yet implemented

[adorton-adobe]

Regarding cache-control --clear (item 1):

The strongest reason to leave this as-is is that it sort of provides an extra layer of confirmation for performing a dangerous action. However, I think it may make more sense to follow the typical pattern of prompting the user for confirmation and perhaps even accepting a -y option to bypass that confirmation prompt.

$ ./frl-proxy cache-control --clear
Are you sure you want to clear the cache? This operation cannot be undone [yN]: Y
cache-control: Cache has been cleared

$ ./frl-proxy cache-control --clear -y
cache-control: Cache has been cleared

[adobeDan]

Great proposal! I completely agree with your reasoning, @adorton-adobe.