Support autoconfigure additional redis instances

[quaff]

It's common to connect multiple redis servers in the sample application, currently It's not easy to reuse RedisAutoConfiguration and LettuceConnectionConfiguration to create multiple redis related beans, I hope spring-boot could handle this just by adding a few properties, for example:

# primary redis
spring.redis:
  host: localhost
  port: 6379

# additional redis instances
spring.redis.additional-instances: foo,bar
foo.redis:
  host: localhost
  port: 6379
  database: 1
bar.redis:
  host: localhost
  port: 6379
  database: 2

default redis related beans should be @Primary

// default redis
@Autowired RedisTemplate redisTemplate;
@Autowired RedisTemplate stringRedisTemplate;

// foo redis
@Autowired RedisTemplate fooRedisTemplate;
@Autowired RedisTemplate fooStringRedisTemplate;

// bar redis
@Autowired RedisTemplate barRedisTemplate;
@Autowired RedisTemplate barStringRedisTemplate;

[quaff]

Actually It's impossible to reuse LettuceConnectionConfiguration since it's not visible.

[philwebb]

This is a general problem with our current design and something that we hope to address more holistically with #21322. I don’t think we’ll be able to tackle the problem in a piecemeal fashion without making things worse. We’d rather keep our existing approach of requiring manual configuration if multiple services are being used until we can find a good general purpose solution.