ecdsa_adaptor: initialize project

This commit adds the foundational configuration and building scripts
and an initial structure for the project.

src/modules/ecdsa_adaptor/Makefile.am.include

@@ -0,0 +1 @@
+include_HEADERS += include/secp256k1_ecdsa_adaptor.h

src/modules/ecdsa_adaptor/main_impl.h

@@ -0,0 +1,12 @@
+/**********************************************************************
+ * Copyright (c) 2020-2021 Jonas Nick, Jesse Posner                   *
+ * Distributed under the MIT software license, see the accompanying   *
+ * file COPYING or http://www.opensource.org/licenses/mit-license.php.*
+ **********************************************************************/
+
+#ifndef SECP256K1_MODULE_ECDSA_ADAPTOR_MAIN_H
+#define SECP256K1_MODULE_ECDSA_ADAPTOR_MAIN_H
+
+#include "include/secp256k1_ecdsa_adaptor.h"
+
+#endif