feat(entrypoint): add support for rotating mnemonic through docker entrypoint

[talalashraf]

The following changes were introduced:

• some issues/recommendations around usage of quotes, leveraging arrays for args, using < instead of piping input etc. came from shellcheck. the file now passes shellcheck
• added a rotate_mnemonic function. This function is called before running the daemon in auto mode if mnemonic exists already
• the function checks for a file called rotate in tofnd home. if found, it will rotate the key and delete the rotate file. if not found it will skip rotation
• this makes it easy to rotate the mnemonic. we simply have to add a file in tofnd home and thats it.

[talalashraf]

this will be irrelevant after we deploy the helm based validators. we will find a better way of doing it.