Add exception type for READONLY errors

[anothertobi]

Feature Request

Add a distinct exception type for READONLY errors so that users can handle these in a clean way.

Is your feature request related to a problem? Please describe

With some Redis setups (e.g. HAProxy abstracting Redis/Sentinel setup), clients are forced to handle failovers by re-creating their connection when they receive READONLY errors.

Describe the solution you'd like

From io.lettuce.core.internalExceptionFactory:

Specific Redis error messages may create subtypes of {@link RedisCommandExecutionException}.

Therefore I'd like to have a RedisReadOnlyException, similar to the existing RedisLoadingException.

Describe alternatives you've considered

Clients can already handle this by checking the message of a RedisCommandExecutionException for READONLY.

Teachability, Documentation, Adoption, Migration Strategy

Users can implement custom code based on the new exception type.