Adds pre-shared key authentication for OpenSSL streams, covering both TLS 1.2 (PSK ciphersuites) and TLS 1.3 (PSK as an external resumed session).

The Openssl\Psk class is following:

namespace Openssl;

final class Psk {
    public const int MAX_PSK_LEN;
    public const int MAX_IDENTITY_LEN;
    public readonly string $psk;
    public readonly ?string $identity;
    public function __construct(string $psk, ?string $identity = null);
}

Two new options under the ssl context:

• psk_client_cb: callable(resource $stream): ?Openssl\Psk
  Called on the client during the handshake to obtain the PSK and identity. Return null to decline PSK. The returned Psk must have a non-null identity.
• psk_server_cb: callable(resource $stream, string $identity): ?Openssl\Psk
  Called on the server with the identity offered by the client. Return null to reject the identity. Only the key is read from the returned Psk.

For TLS 1.3 servers a default session_id_context is installed when the user does not supply one, since OpenSSL processes PSK as session resumption and would otherwise reject the constructed SSL_SESSION with SSL_R_SESSION_ID_CONTEXT_UNINITIALIZED under the default SSL_VERIFY_PEER configuration. An explicit session_id_context still takes precedence.

The RFC is not planned unless there are objections as it mirrors the previous Session proposal including all API decisions from it.