Entity cache: only cache data if the subgraph returns a Cache-Control header or if a TTL is configured

[Geal]

the intent was to cache data from subgraphs only if a Cache-Control header was returned, but there is currently a bug, where all of the data is cached even when the header is not present, and if the TTL is not configured, the data will not expire.
There is also confusion on what should be done if the TTL is configured. So, to nail down how caching should behave:

• if not configured on any subgraphs, do not cache anything
• enabled option, boolean, activates caching, defaults to false. It can be overridden per subgraph, so if it is set to true and a subgraph sets it to false, then all subgraph will be cached except for that specific subgraph. At this point caching is still opt-in on the part of the subgraph, it has to provide a Cache-Control header
• ttl option, duration. A global TTL can be configured under the redis option. It can also be overridden per subgraph. If a subgraph returns a Cache-Control header without a TTL, then we will get that TTL from configuration. If the header specifies a max-age, then we will use that for expiration
• add an option always_cache, to cache responses even when the subgraph does not provide a Cache-Control header

We also need a reasonable default TTL for caching, to make sure we can't have data stored indefinitely

In the future, once the router has more ability to recognize which parts of the schema should be cached:

• if enabled, globally or per subgraph:
• if the subgraph does not provide a Cache-Control header, rely on the indications from the schema
• if the subgraph provides a Cache-Control header, follow what the header says