Add initial implementation of custom cache key

[veeshi]

Initail implementation for #25

Some things of note:

• The cache_key closure is wrapped in an Arc to allow the HttpCache struct to be cloned.
• The CacheManager function signatures were changed to take just the cache key.
• An override cache key parameter was added to the HttpCacheOptions.create_cache_key function to not change the behaviour for is_get_head.
• All tests are passing and a test was added to the client_reqwest tests called custom_cache_key.

[06chaynes]

Created a develop branch, probably should have a long time ago but 🤷

Set that branch as the target and will hopefully have some time later to check out the changes

[06chaynes]

Thanks for your patience on this, had a birthday the other day so just getting back around to this. Changes look good to me!