dd4hep: add g4hepem patch

[wdconinc]

Briefly, what does this PR introduce? Please link to any relevant presentations or discussions.

This PR backports the G4HepEm plugin to DD4hep 1.36.

What is the urgency of this PR?

• High (please describe reason below)
• Medium
• Low

What kind of change does this PR introduce?

• Bug fix (issue #__)
• New feature (issue: DD4hep G4HepEm plugin)
• Optimization (issue #__)
• Updated documentation
• other: __

Please check if any of the following apply

• This PR introduces breaking changes. Please describe changes users need to make below.
• This PR changes default behavior. Please describe changes below.
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[Copilot]

Pull request overview

This PR updates the dd4hep Spack overlay package to support building the G4HepEm plugin for DD4hep 1.36 via an upstream patch and a new build variant.

Changes:

• Add a +g4hepem variant (default enabled) for DD4hep @1.36: when +ddg4 is enabled.
• Add a conditional dependency on g4hepem when +g4hepem.
• Apply a backport patch for DD4hep @1.36:1.37 and wire the variant into CMake via DD4HEP_USE_G4HEPEM.

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