Add page size limit

[pulpn0ir]

Adds a means to prevent server overloads by limiting the maximum amount of elements returned in a single page.

Application and Request have been extended with setters for page size limits. Set limits using the newly introduced PageLimit type.

// Set page size limit for the current request. Default is `nil`, which means no limit. 
request.fluent.pagination.pageSizeLimit = 3

// Setting the request-level limit to `nil` will cause the application-level limit to be used instead. 
request.fluent.pagination.pageSizeLimit = nil

// Use `.noLimit` if you intend to override a limit set on application level. 
request.fluent.pagination.pageSizeLimit = .noLimit

// Set application-wide page size limit. Default is `nil`, which means no limit. 
application.fluent.pagination.pageSizeLimit = 3

This PR depends on: https://github.com/vapor/fluent-kit/pull/412

[siemensikkema]

The tests are failing due to unrelated changes.

• Swift nightly 5.2: error: CacheTests.testCacheSet : threw error "invalidValue("bar", Swift.EncodingError.Context(codingPath: [], debugDescription: "Top-level String encoded as string JSON fragment.", underlyingError: nil))" There seems to be a regression in JSON encoding here. Strangely, the nightly 5.3 builds fine.
• Swift nightly: a compiler crash during type checking of EnumBuilder.swift

Besides reporting these I think we should update CI to not depend on nightly builds. @gwynne @0xTim agreed?

[0xTim]

@siemensikkema agree, for most things we should probably be testing with lowest supported version (5.2 release) and current release (5.4). It might be beneficial to test against latest master to catch Swift regressions but there's no point testing against a nightly build of a released Swift version

[VaporBot]

These changes are now available in 4.3.0