Allow DevTools to load optional config file for local development overrides

[asaikali]

Introduce support in Spring Boot DevTools to automatically and optionally load a application-devtools.yml (or .properties) file as a local override configuration source during development.

Motivation
When developing Spring Boot applications, it is common to need local-only configuration such as:

• Local service endpoints or ports
• Developer-specific API keys or credentials
• Debug or logging tweaks
• Feature toggles or secrets

Currently, developers must choose between:

• Creating profile-specific overrides (e.g., application-dev.yml) but there is no standard convention for what this file is called, so every project does something different, making it a bespoke effort to configure locally runnable boot app.
• Using spring.config.import in application.yml, which adds complexity and clutter to the main configuration, and may accidentally leak dev-only paths into production environments.

Proposed Behavior
If DevTools is present on the classpath, Spring Boot would automatically and optionally import a config file named:

application-devtools.yml

(or .properties), if it exists in the same ___location(s) as the other config files (e.g., src/main/resources, or external locations).

• It would not require any spring.config.import entry.
• It would be gracefully ignored if not present.
• Developers can .gitignore it by default:

  application-devtools.yml

• IDEs and CLI builds using DevTools (bootRun) would pick it up transparently.

Benefits

• Safer and cleaner than custom spring.config.import hacks.
• Avoids bloating application.yml with dev-only logic.
• Encourages best practices: local config override without leaking to production.
• Improves parity with how spring-boot-devtools already supports dev-only beans and runtime behavior.

Thanks for considering this enhancement! It would make DevTools even more useful and offer a convention that makes all spring boot developers lives easier.

[wilkinsona]

Have you considered using the global settings file? Beyond that, this is a duplicate of #42066.