Provide convenience sql functions for manipulating cache settings

[binaek]

Right now, the only way to tweak the cache is to INSERT into the steampipe_internal.steampipe_settings table - which is used by the FDW to tweak cache settings.

This is inconvenient.

We should have SQL functions to help with cache manipulation.

It should be set_cache(value)
where value is the same as the .cache meta-command?
set_cache("on")
set_cache("off")
set_cache("clear")