Allow application properties to be contributed from module jars

[philwebb]

Currently application.properties and application.yaml files are loaded from the classpath and the first match wins. This makes it hard to bundle common properties into jar files. It would be nice if we could find a way for jars to also contribute properties without needing to implement a EnvironmentPostProcessor.

There are a few things we need to be careful of:

• What order to we load the resources and how do we make sure a developer can override a contributed value
• We don't want to increase startup time (for example by doing a full classpath search)
• We can't break back-compatibility
• We need to be mindful of spring.config.import=classpath:

[mbhave]

Ordering will be tricky with this one. If two jars happen to provide application.properties files with the same value, we need a way to determine which one will win.

[alex-lx]

Would using an order property in application.properties be a simple way?

[philwebb]

We should look into the use-case behind #25033 when we fix this one.

[stategen]

@philwebb & @wilkinsona ,thanks for your great work!

--spring.config.intergration-location=classpath*:/sink//[application.yml], in my pull request #25082, similar #25080

the dependency jar's configs just to reduce the duplicate or mistake works by the the project who use it.
so the rule: lowest order and can be overrided by any, not care conflicts(use map value),not overrides higher/outside value, limit 2 wildcards ,endswith '*/' or filename and filtered by '/fold/' .
the rule will never break back-compatibility.
and with a new arg to ensure never break back-compatibility.
In my real project（240 jars） with the update i pulled, the startup time is not slow,just normally.
I perfer to use spring in xml(<resource import="classpth*:">), but more and more cloud-framework only provide with spring-boot'style config,there is really no back-way to use spring-boot

[stategen]

I perfer to use spring in xml() = I perfer to use spring in xml(< resource import="classpath:*">)

[wilkinsona]

We should also consider #25084 when looking at this.