Allow binder to compose collections from multiple sources

Currently the Binder class has a "first wins" policy for binding lists. From the docs:

When lists are configured in more than one place, overriding works by replacing the entire list.

This policy has worked well, however, it also causes problems such as #41669 where users really want to merge lists together.

I think it would be a mistake to attempt to compose lists from elements in different property sources, however, we might be able to offer a feature where distinct lists could be merged together.

Proposal

When property values are defined, the property name could include an additional indicator that is used to determine how the values should be merged. The indicator would be (+) for addAll and (-) for removeAll:

This example shows an auto-configure exclude:

.properties

spring.autoconfigure.exclude=org.springframework.boot.autoconfigure.elasticsearch.ElasticsearchClientAutoConfiguration
#---
spring.config.activate.on-profile: noredis
spring.autoconfigure.exclude(+)=org.springframework.boot.autoconfigure.data.redis.RedisAutoConfiguration

.yaml

spring:
  autoconfigure:
    exclude: org.springframework.boot.autoconfigure.elasticsearch.ElasticsearchClientAutoConfiguration
---
spring:
  config:
    activate:
      on-profile: noredis
  autoconfigure:
    exclude(+): org.springframework.boot.autoconfigure.data.redis.RedisAutoConfiguration

More complex examples are:

.properties

my.list-of-names(+)[0].first=Spring
my.list-of-names(+)[0].last=Boot
my.list-of-names(+)[1].first=Spring
my.list-of-names(+)[1].last=Framework

.yaml

my:
  list(-):
    - remove
    - stuff

Limitations

It will be hard to support this syntax with environment variables.

Start by reading the Binder class and the documented list-binding behavior, then trace how property names could represent the proposed (+) and (-) indicators. Compare the properties and YAML examples, account for the environment-variable limitation, and establish tests and documentation that define when merging is complete.