Add page size limit

This patch was authored by @pulpn0ir and released by @siemensikkema.

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: vapor/fluent-kit#412