Add support for caching introspection results (and other plugin values)

[benjie]

Should support generic storage for each plugin, the plugins should manage their own version information (e.g. PgIntrospectionPlugin might use a hash of the introspection query, or simply the version number of the plugin, others may factor in the arguments received).

Failure should be hard - e.g. if the options don't match but you're asked to import a cache then it should throw and should be called again without an input cache so that a fresh cache can be generated.

[benjie]

Filing this as a bug because we need it before we can release PostGraphile v4. It's not really a bug, it's a missing feature.

[benjie]

https://github.com/graphile/graphile-build/issues/118