ecdsa_adaptor: initialize project

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

include/secp256k1_ecdsa_adaptor.h

@@ -0,0 +1,19 @@
+#ifndef SECP256K1_ECDSA_ADAPTOR_H
+#define SECP256K1_ECDSA_ADAPTOR_H
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+/** This module implements single signer ECDSA adaptor signatures following
+ *  "One-Time Verifiably Encrypted Signatures A.K.A. Adaptor Signatures" by
+ *  Lloyd Fournier
+ *  (https://lists.linuxfoundation.org/pipermail/lightning-dev/2019-November/002316.html
+ *  and https://github.com/LLFourn/one-time-VES/blob/master/main.pdf).
+*/
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif /* SECP256K1_ECDSA_ADAPTOR_H */