Preserve POSIX file permissions for entries added from classesDirectory

## Use case

A project builds a JAR on Linux and later extracts that JAR on Linux as part of assembling a distributable artifact. Some resources are executable shell scripts. The extracted scripts should retain their source POSIX permissions, such as `0755`, so they can be invoked directly.

## Current behavior

With Maven Jar Plugin 3.5.1 on Linux:

1. A resource file under `src/main/resources` is mode `0755`.
2. The copied file under `target/classes` remains mode `0755`.
3. After `mvn package`, the corresponding JAR entry has Unix mode `100644`.
4. Extracting the JAR on Linux produces a non-executable file.

The plugin creates a `DefaultFileSet` for `classesDirectory` without resource mode attributes, so Plexus Archiver falls back to its default file mode.

## Requested behavior

When creating a JAR on a POSIX filesystem, preserve the POSIX file permissions of entries added from the classes directory (or provide an opt-in configuration that does so). This would allow a JAR created on Linux and extracted on Linux to retain executable resource scripts.

This is distinct from the mode of the generated JAR file itself; it concerns the Unix mode metadata of entries inside the JAR.

## Reproduction

```sh
chmod 755 src/main/resources/hooks/session-start.sh
mvn package
zipinfo -l target/example.jar | grep session-start.sh
```

Expected entry mode: `-rwxr-xr-x` (`100755`).

Actual entry mode: `-rw-r--r--` (`100644`).

Start at the JAR creation path that creates a DefaultFileSet for classesDirectory, then trace how Plexus Archiver assigns entry modes when resource attributes are absent. Reproduce with the chmod and mvn package commands, and inspect target/example.jar with zipinfo -l. Done means a 0755 source script appears as 100755 in the JAR on POSIX systems without changing the generated JAR file's own mode.

Written by the indexing model from the issue text.