Add get_rpath() method to ElfContainer

[nichmor]

Summary

• Add get_rpath() method to ElfContainer that returns RPATH/RUNPATH entries as Vec<String>
• Enables maturin to use arwen instead of goblin for reading rpath values

Context

This change supports PyO3/maturin#2932 which replaces patchelf with arwen. The PR currently has a get_rpath function using goblin directly with a TODO comment "Unify goblin and object usage."

With this method, maturin can replace:

// Before (goblin):
match goblin::Object::parse(&contents) {
    Ok(goblin::Object::Elf(elf)) => {
        let rpaths = if !elf.runpaths.is_empty() { elf.runpaths } else { elf.rpaths };
        Ok(rpaths.iter().map(|r| r.to_string()).collect())
    }
}

// After (arwen):
let elf = ElfContainer::parse(&contents)?;
Ok(elf.get_rpath())

Test plan

• cargo check passes

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