Customizable pom.xml path for Java transformation

## Problem

Currently with Amazon Q plugin, `/transform` will only identity the `pom.xml` in the root directory of current workspace. In some complex scenarios, there is no `pom.xml` in the root directory, they are in many sub folders instead.

Besides, users may want to upgrade one module at a time, instead of the whole project. Currently users have to open a sub module as the workspace.

## Expected behavior

When scanning for java projects, use `pom.xml` in the root directory as default but allow user to customize it.

Start by tracing the Java project scanning used by the /transform command and how it selects pom.xml from the workspace root. Define where a user-configurable pom.xml path belongs, preserve the root pom.xml as the default, and verify that selecting a module transforms only that module.

Written by the indexing model from the issue text.