topdown/http: validate force_cache parameters

Earlier the validation of the force_cache parameters was performed
after creating the inter-query cache object. So if 'force_cache' was
set and 'force_cache_duration_seconds' was not provided, OPA in server mode
would correctly return an error but OPA running in repl mode would not
return an error and simply would fall back to using the intra-query cache.

This commit updates how the 'force_cache' parameters are validated so that
consistent behavior is seen in both repl and server modes.

Signed-off-by: Ashutosh Narkar <anarkar4387@gmail.com>