Add content.config to the context object.

[timleslie]

When writing hooks, access control function, virtual fields, or any other custom functionality which is executed within a resolver, it can be helpful to be able to access information about the full keystone system itself. Which there are various workarounds to this at the moment, none of them are particularly elegant, and they don't lend themselves to consistent, easily explained patterns.

To solve for this, we will add a new object to the context object which is passed to all resolver functions. context.config will mirror the config object exported from keystone.ts, but with some minor adjustments. In particular, any default or computed values which are determined during Keystone start up will be populated. This will give consumers of the object access to the true values being used in a consistent way.