Add separate private qchem functions for external backends

[soranjh]

Context:
The molecular_hamiltonian function currently supports three backends: the in-built differentiable Hartree-Fock backend, the pyscf backend, and the openfermion backend. This PR creates dedicated functions for calling these backends and puts them in proper modules. These changes are not user-facing but help with easier maintenance and extension of qchem functions.

Description of the Change:

1. Create _openfermion_hamiltonian function for building a Hamiltonian with the openfermion backend.
2. Move the molecular_hamiltonian function to hamiltonian.py.

Benefits:
Easier maintenance and extension of qchem functions.

Possible Drawbacks:
None

Related GitHub Issues:
None

[soranjh]

[sc-59834]

[github-actions]

Hello. You may have forgotten to update the changelog!
Please edit doc/releases/changelog-dev.md with:

• A one-to-two sentence description of the change. You may include a small working example for new features.
• A link back to this PR.
• Your name (or GitHub username) in the contributors section.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.67%. Comparing base (22342c9) to head (f9aea69).
Report is 254 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5553      +/-   ##
==========================================
- Coverage   99.67%   99.67%   -0.01%     
==========================================
  Files         414      414              
  Lines       39463    39172     -291     
==========================================
- Hits        39335    39043     -292     
- Misses        128      129       +1     

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[ddhawan11]

Thanks @soranjh. I only have a couple of minor comments.

[ddhawan11]

Thanks @soranjh. Looks good to me.

[obliviateandsurrender]

Good to go, just a couple of minor points.