[MSHADE-154] - Add ability to shade an attached primary artifact

[wilx]

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Summary

Add an optional inputClassifier parameter that lets the Shade Plugin use an
artifact attached to the current project as its primary input. When the
parameter is omitted or blank, the plugin continues to use the project's main
artifact.

Root cause

The plugin always used MavenProject#getArtifact() as the primary input. A
plugin could produce and attach a classified JAR before the shade execution,
but Shade had no way to select it. Projects that intentionally produced only
the classified artifact consequently failed because the main artifact did not
exist.

Implementation

The selected attachment is found directly in the current project's attached
artifacts. Missing and ambiguous classifier selections fail with actionable
messages. The selected artifact is then used consistently for artifact-set
selection, archive filtering, and minimization.

When the shaded artifact is not attached separately, it retains the existing
output semantics and becomes the project's main artifact. The original
classified input remains attached and is not overwritten.

The new integration test covers selection of a classified input, archive
filtering, dependency minimization, preservation of the input attachment, and
installation of the shaded JAR as the main artifact. Unit tests cover default,
successful, missing, and ambiguous selection.

Verification

• mvn clean verify — 76 tests passed
• mvn -Prun-its clean verify — 84 integration tests passed; 2 existing
  JRE-dependent tests were skipped under Java 8
• The focused reproducer also passed under Maven 4.0.0-rc-5

Fixes #516.

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