Swift 5.5: Remove Factory structs

[adam-fowler]

With Swift 5.5 we don't need to create a separate Factory struct for CredentialProvider and RetryPolicy. The following will now work

struct EmptyCredentialProvider: CredentialProvider {}
extension CredentialProvider where Self == EmptyCredentialProvider {
    static var empty: Self { return EmptyCredentialProvider() }
}
func test(_ base: CredentialProvider) {
}
test(.empty)