Support Maven mirrors in generated settings.xml #1287

Description

Add a new input mvn-mirrors to the Maven settings generation, analogous to mvn-repositories and mvn-plugin-repositories.

• Format: multiline list of mirror-id:mirror-url:mirror-of.
• mirror-of is the Maven <mirrorOf> pattern, e.g. *, central or external:*.
• Each entry is written as a <mirror> into the <mirrors> section of the generated settings.xml.
• Mirror IDs should match mvn-server-credentials IDs for authentication, as with mvn-repositories.

Example:

- uses: actions/setup-java@v6
  with:
    distribution: temurin
    java-version: '21'
    mvn-server-credentials: |
      private:PRIVATE_REPOSITORY_USERNAME:PRIVATE_REPOSITORY_TOKEN
    mvn-repositories: |
      private:https://maven.example.com/releases:false
    mvn-plugin-repositories: |
      private:https://maven.example.com/releases:false
    mvn-repositories-include-central: false
    mvn-mirrors: |
      private:https://maven.example.com/releases:*

Generated output:

<mirrors>
  <mirror>
    <id>private</id>
    <url>https://maven.example.com/releases</url>
    <mirrorOf>*</mirrorOf>
  </mirror>
</mirrors>

Justification

mvn-repositories and mvn-plugin-repositories only generate <repositories> and <pluginRepositories> in a profile. These are added on top of the repositories already declared elsewhere. Maven therefore still contacts:

• Maven Central from the Super POM.
• Any repository declared in a project or parent POM.

Even with mvn-repositories-include-central: false, these remain active. Builds in corporate environments, where all artifacts must come from an internal repository, can still reach external hosts or fail on them.

Only a <mirror> with <mirrorOf>*</mirrorOf> reliably redirects every repository, including central and POM-declared ones, to the internal repository. The action cannot configure this today.

The current workaround is a custom step that patches ~/.m2/settings.xml after setup-java has run, for example:

perl -0pi -e 's#<servers>#<mirrors>...</mirrors><servers>#' ~/.m2/settings.xml

The other option is a hand-written settings.xml with overwrite-settings: false. Both defeat the purpose of the mvn-repositories and mvn-server-credentials inputs. A native mvn-mirrors input removes the need for this patching.