Use const char pointer in redismodule.h as far as possible (#10064)

When I used C++ to develop a redis module. i  used `string.data()` as the second parameter `ele`
of  `RedisModule_DigestAddStringBuffer`, but there is a warning, since we never change the `ele`,
i think we should use `const char` for it.

This PR adds const to just a handful of module APIs that required it, all not very widely used.
The implication is a breaking change in terms of compilation error that's easy to resolve, and no ABI impact.
The affected APIs are around Digest, Info injection, and Cluster bus messages.

tests/modules/datatype2.c

@@ -627,7 +627,7 @@ void MemAllocDigest(RedisModuleDigest *md, void *value) {
 
         struct MemBlock *block = mem;
         while (block) {
-            RedisModule_DigestAddStringBuffer(md, (unsigned char *)block->block, BLOCK_SIZE);
+            RedisModule_DigestAddStringBuffer(md, (const char *)block->block, BLOCK_SIZE);
             block = block->next;
         }
     }