[metadata] Initial support for MONO_CALL_UNMANAGED

A MethodRefSig or a StandaloneSig can have the calling convention "unmanaged"
which is the default platform calling convention with optional modifiers
encoded in the modopts of the return type.

See dotnet/roslyn#39865 (comment)
and dotnet/runtime#34805

Contributes to dotnet/runtime#38480

mono/metadata/metadata.c

@@ -2342,6 +2342,7 @@ mono_metadata_parse_method_signature_full (MonoImage *m, MonoGenericContainer *c
 	case MONO_CALL_STDCALL:
 	case MONO_CALL_THISCALL:
 	case MONO_CALL_FASTCALL:
+	case MONO_CALL_UNMANAGED:
 		method->pinvoke = 1;
 		break;
 	}

mono/metadata/metadata.h

@@ -37,7 +37,11 @@ typedef enum {
 	MONO_CALL_STDCALL,
 	MONO_CALL_THISCALL,
 	MONO_CALL_FASTCALL,
-	MONO_CALL_VARARG
+	MONO_CALL_VARARG = 0x05,
+	/* unused, */
+	/* unused, */
+	/* unused, */
+	MONO_CALL_UNMANAGED = 0x09, /* default unmanaged calling convention, with specifics encoded in modopts */
 } MonoCallConvention;
 
 /* ECMA lamespec: the old spec had more info... */