Add COSE key and sign1 rendering. (#83)

Implement a subset of COSE data object rendering. Specifically:
- COSE_Key with support for EC keys with curves p256, p384, and ed25519.
- COSE_Sign1.

The COSE_Sign1 method also constructs a gather list that represents the
Sig_structure used for signing.

See RFC9052.

Author: werwurm

CMakeLists.txt

@@ -94,6 +94,7 @@ set(LIBNAT20_SOURCES
     # Add the core library source files here.
     src/core/asn1.c
     src/core/cbor.c
+    src/core/cose.c
     src/core/functionality.c
     src/core/oid.c
     src/core/stream.c
@@ -110,6 +111,7 @@ set(LIBNAT20_PUB_HEADERS
     include/nat20/asn1.h
     include/nat20/cbor.h
     include/nat20/constants.h
+    include/nat20/cose.h
     include/nat20/crypto.h
     include/nat20/endian.h
     include/nat20/error.h
@@ -147,6 +149,7 @@ set(LIBNAT20_TEST_SOURCES
     # Add test source files here.
     src/core/test/asn1.cpp
     src/core/test/cbor.cpp
+    src/core/test/cose.cpp
     src/core/test/functionality.cpp
     src/core/test/oid.cpp
     src/core/test/stream.cpp