Vapor 4 support + custom cache keys

[madsodgaard]

This PR adds Vapor 4 support with the following API:

app.gatekeeper.config = .init(maxRequests: 10, per: .second)

Other than that it's possible to use a custom cache, but by default it uses the Vapor 4 in memory cache.

app.gatekeeper.caches.use(.memory)

This PR also adds GatekeeperKeyMaker which is a protocol used to define the cache key for a specific Request this solves: #3
By default it uses GatekeeperHostnameKeyMaker which uses the request hostname to generate the cache key

app.gatekeeper.keyMakers.use(.hostname)

I still need to update the README, but the code should be ready for review.

[siemensikkema]

@madsodgaard Thank you for this well-crafted PR! I think the translation to Vapor 4 is very well done and the added customizability is more than welcome!
I checked out the code and made some small fixes in this branch https://github.com/nodes-vapor/gatekeeper/tree/feature/vapor4-PR-review. I find doing PR-reviews on such large changes a bit cumbersome. If you agree with the changes you can just merge that branch in (or cherry-pick those commits you agree with). With that the only thing really missing should be the README. Do you mind updating that to show the new syntax and features? 🙏🏻

[madsodgaard]

@siemensikkema Great to hear! I'll get that branch merged and update the README within a few days.

[madsodgaard]

@siemensikkema Merged your changes and updated the README. Should be ready for review!

[siemensikkema]

Thanks for updating. We're nearly there! Just a few nits in relation to the README and I discovered that .swiftpm has been committed. Could you remove that and add .swiftpm to .gitignore?

[madsodgaard]

@siemensikkema Done 😄

[siemensikkema]

Thank you so much! I'll release this in a minute 👌🏻