refs https://github.com/TryGhost/Toolbox/issues/522
- API-level response caching allows to cache responses bypassing the "pipeline" processing
- The main usecase for these caches is caching GET requests for expensive Content API requests
- To enable response caching add a "cache" key with a cache instance as a value, for example for posts public cache configuration can look like:
```
module.exports = {
docName: 'posts',
browse: {
cache: postsPublicService.api.cache,
options: [ ...
```
- if the API controller endpoint is a function, we early return as we
expect the function to handle the response but we still ended up
calculating the headers beforehand, only to be thrown away
- this commit moves the header fetching code down in the flow so it's
only executed when needed
- this doesn't really have a big effect for us because 99% of our
controllers follow the object pattern
refs https://github.com/TryGhost/Toolbox/issues/363
- this API framework is standalone and should be pulled out into a
separate package so we can define its boundaries more clearly, and
promote better testing of smaller parts
