vwal: introduce iVersion for being future-proof

Similarly to how other interfaces work, the version number in WAL methods
lets the user know which functions are supported, and which aren't yet.

ext/vwal/vwal.c

@@ -116,6 +116,7 @@ static void v_get_wal_pathname(char *buf, const char *orig, int orig_len) {
 __attribute__((__visibility__("default")))
 void libsql_register_vwal() {
   static libsql_wal_methods methods = {
+    .iVersion = 1,
     .xOpen = v_open,
     .xClose = v_close,
     .xLimit = v_limit,

src/wal.c

@@ -4111,6 +4111,7 @@ libsql_wal_methods *libsql_wal_methods_find(const char *zName) {
     zName = "default";
   }
   if (methods_head == NULL && strncmp(zName, "default", 7) == 0) {
+    methods.iVersion = 1;
     methods.xOpen = sqlite3WalOpen;
     methods.xClose = sqlite3WalClose;
     methods.xLimit = sqlite3WalLimit;

src/wal.h

@@ -33,6 +33,7 @@
 typedef struct Wal Wal;
 
 typedef struct libsql_wal_methods {
+  int iVersion; /* Current version is 1, versioning is here for backward compatibility *.
   /* Open and close a connection to a write-ahead log. */
   int (*xOpen)(sqlite3_vfs*, sqlite3_file* , const char*, int no_shm_mode, i64 max_size, struct libsql_wal_methods*, Wal**);
   int (*xClose)(Wal*, sqlite3* db, int sync_flags, int nBuf, u8 *zBuf);