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.

1 files changed, 2 insertions, 2 deletions

diff --git a/src/server.h b/src/server.h
index 8c8cfcf9b..3978201a7 100644
--- a/src/server.h
+++ b/src/server.h
@@ -3367,8 +3367,8 @@ void applyWatchdogPeriod();
 void watchdogScheduleSignal(int period);
 void serverLogHexDump(int level, char *descr, void *value, size_t len);
 int memtest_preserving_test(unsigned long *m, size_t bytes, int passes);
-void mixDigest(unsigned char *digest, void *ptr, size_t len);
-void xorDigest(unsigned char *digest, void *ptr, size_t len);
+void mixDigest(unsigned char *digest, const void *ptr, size_t len);
+void xorDigest(unsigned char *digest, const void *ptr, size_t len);
 int populateSingleCommand(struct redisCommand *c, char *strflags);
 void commandAddSubcommand(struct redisCommand *parent, struct redisCommand *subcommand);
 void populateCommandMovableKeys(struct redisCommand *cmd);