Add Credentials Provider to allow short(er) lived credentials.

[holomekc]

Motivation:

At the moment the implementation only allows one hard coded user and password. In theory it would be possible to override the getPassword method of RedisOptions, but this is pretty hacky and also blocks the thread in case providing the credentials (or here just the password) takes a longer time.

With the provided changes it is possible to dynamically adjust the credentials used for a new connection. The rule is: credentials provider > URI > RedisOptions (if available). This allows to use short lived credentials or allow to rotate credentials and update them without reconfiguration. In the Issues section I saw that somebody asked for AWS credentials. This should be possible here in multiple ways. E.g. using IAM auth added for ElastiCache recently, or extracing credentials from AWS SecretsManager (with rotation), etc.

Questions: I hope I managed to create some useful tests. I was ok with creating the adjustments for RedisConnectionManager, but I am a bit confused regarding the RedisClusterConnection class. I basically searched for getPassword() usage and these are the only two locations. But the later one is never considering the username. This looks a bit strange to me. I just adjusted it so that it still just considers the username and skips the username information there.