[nikhilnl] Spack exercise

[Nikhil-4595]

This pull request adds the complete Spack package definition for the spack-exercise project as required by the course assignment.

Summary of Changes:

• Added package.py implementing the Spack recipe for the CMake-based project.
• Added versions 0.1.0, 0.2.0, and 0.3.0 with the correct SHA256 checksums.
• Added maintainer entry.
• Added MIT license information.
• Added build requirements:
  depends_on("cxx", type="build")
depends_on("c", type="build")
• Added versioned dependencies:
  depends_on("boost@1.65.1:", when="@0.2.0:")
depends_on("yaml-cpp@0.7.0:", when="@0.3.0")

Validation Performed:

• spack-exercise@0.1.0 builds, installs, loads, and runs correctly inside the provided Docker container.
• spack-exercise@0.2.0 builds, installs, loads, and runs correctly and shows Boost functionality.
• spack-exercise@0.3.0 builds, installs, loads, and runs correctly and shows yaml functionality.
• Version and dependency resolution behaves as expected using Spack’s concretizer.
• The package metadata (homepage, license, maintainer, versions) displays correctly under:
  spack info spack-exercise

All expected functionality has been verified inside the course environment, and the package behaves as required.

[IshaanDesai]

Good 👍