Add RM_RdbLoad and RM_RdbSave module API functions (#11852)

Add `RM_RdbLoad()` and `RM_RdbSave()` to load/save RDB files from the module API. 

In our use case, we have our clustering implementation as a module. As part of this
implementation, the module needs to trigger RDB save operation at specific points.
Also, this module delivers RDB files to other nodes (not using Redis' replication).
When a node receives an RDB file, it should be able to load the RDB. Currently,
there is no module API to save/load RDB files. 


This PR adds four new APIs:
```c
RedisModuleRdbStream *RM_RdbStreamCreateFromFile(const char *filename);
void RM_RdbStreamFree(RedisModuleRdbStream *stream);

int RM_RdbLoad(RedisModuleCtx *ctx, RedisModuleRdbStream *stream, int flags);
int RM_RdbSave(RedisModuleCtx *ctx, RedisModuleRdbStream *stream, int flags);
```

The first step is to create a `RedisModuleRdbStream` object. This PR provides a function to
create RedisModuleRdbStream from the filename. (You can load/save RDB with the filename).
In the future, this API can be extended if needed: 
e.g., `RM_RdbStreamCreateFromFd()`, `RM_RdbStreamCreateFromSocket()` to save/load
RDB from an `fd` or a `socket`. 


Usage:
```c
/* Save RDB */
RedisModuleRdbStream *stream = RedisModule_RdbStreamCreateFromFile("example.rdb");
RedisModule_RdbSave(ctx, stream, 0);
RedisModule_RdbStreamFree(stream);

/* Load RDB */
RedisModuleRdbStream *stream = RedisModule_RdbStreamCreateFromFile("example.rdb");
RedisModule_RdbLoad(ctx, stream, 0);
RedisModule_RdbStreamFree(stream);
```

1 files changed, 1 insertions, 0 deletions

diff --git a/src/rdb.h b/src/rdb.h
index dc096fd8b..234bde221 100644
--- a/src/rdb.h
+++ b/src/rdb.h
@@ -157,6 +157,7 @@ int rdbLoad(char *filename, rdbSaveInfo *rsi, int rdbflags);
 int rdbSaveBackground(int req, char *filename, rdbSaveInfo *rsi, int rdbflags);
 int rdbSaveToSlavesSockets(int req, rdbSaveInfo *rsi);
 void rdbRemoveTempFile(pid_t childpid, int from_signal);
+int rdbSaveToFile(const char *filename);
 int rdbSave(int req, char *filename, rdbSaveInfo *rsi, int rdbflags);
 ssize_t rdbSaveObject(rio *rdb, robj *o, robj *key, int dbid);
 size_t rdbSavedObjectLen(robj *o, robj *key, int dbid);