Add a new inter-query cache to cache responses across queries

This commit adds a new inter-query cache that built-in
functions can use to cache responses across queries.

The OPA config includes a new "caching" field that can be used
to set the size of the cache. By default there is no limit.

This change also updates `http.send` to optionally utilize the
inter-query cache.

Fixes #1753

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