Refactored how CMake passes version & settings info

- keys can now be passed per-build passing -DLAIKA_PUBKEY=[pubkey] & -LAIKA_PRIVKEY=[pubkey] to `cmake -B`
- if those definitions aren't passed, the default public & private key will be used (for quick testing)

tools/genkey/src/main.c

@@ -5,7 +5,7 @@
 #include "lrsa.h"
 
 #define DATA "Encryption/Decryption test passed!\n"
-#define DATALEN 35
+#define DATALEN 36
 #define CIPHERLEN crypto_box_SEALBYTES + DATALEN
 
 int main(int argv, char **argc) {