Change package API to hide redis client library used internally

[hibiken]

Is your feature request related to a problem? Please describe.
Currently, both Client and Background constructor takes go-redis/redis/v7 as a parameter. This will become an issue when there is a new version or if we want to switch to another redis-client library. The change will require major version update, which we don't want to do just for a redis-client library change.

Describe the solution you'd like
We should abstract this away from asynq package users and take RedisClientOption as a parameter.

Describe alternatives you've considered
None so far.