SpringBoot Support autoconfigure additional redis instances

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;

Comment From: quaff

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

Comment From: 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.