Add default file caching

[bshaffer]

For any company running at scale, a default file cache is pretty much a requirement to ensure that the client libraries and auth library function in a performant way. Creating a file cache which works out of the box (and by default) would be a huge help to our customers which are running these libraries at scale.

Rationale: customers have had to create wrappers which implement their custom caching, and this has caused headache, when the configuration for these caches should be intuitive enough and easy enough to implement that they can happen out of the box without handholding or wrapper classes.

For examples of existing FileCache, see php-cache/filesystem-adapter, which uses the FilesystemCachePool, and is implemented in the BaseTest class of our Apiary client library.

• Follow the PHP-FIG PSR-6 caching interfaces
• Make sure that the getCacheKey methods are unique and safe
• Verify with testing and benchmarks that a PHP application receiving multiple requests and handling multiple processes will handle them in a scalable way (e.g Metadata Server credentials, JWT credentials, etc).