Use swift-certificates to generate CSR

[florentmorin]

X509 CSR generation has been replaced by swift-certificates implementation.

Some helpers have also been added.

New implementations:

extension AcmeSwift {

    public struct OrdersAPI {

        // Modified to use `swift-certificate` CSR format
        public func finalize(order: AcmeOrderInfo, withCsr: CertificateSigningRequest) async throws -> AcmeOrderInfo

        // Helper with ECDSA format
        public func finalizeWithEcdsa(order: AcmeOrderInfo, subject: String? = nil, domains: [String]) async throws -> (Certificate.PrivateKey, CertificateSigningRequest, AcmeOrderInfo)

        // Helper with RSA format
        public func finalizeWithRsa(order: AcmeOrderInfo, subject: String? = nil, domains: [String]) async throws -> (Certificate.PrivateKey, CertificateSigningRequest, AcmeOrderInfo)
    }
}

Resolves #1

[m-barthelemy]

That's a very nice contribution, really appreciate it!