Add null qubit

[AmintorDusko]

Context: Implement a Catalyst runtime plugin that mocks out all functions in the QuantumDevice interface and implement a PennyLane device plugin (implemented in qjit_device.py) providing access to the Null runtime device.

Description of the Change:

• Rename the device from "dummy" to "null".
• Make sure the device is up to date with the latest version of the QuantumDevice interface.
• Ensure proper unit testing of the device in C++.
• Package and ship this plugin for both development builds and wheels.

Benefits:
Such a device will be useful for a few reasons:

• "execute" programs without needing to install additional plugins
• Simplify and speed up integration tests when numerical results aren't needed
• Benchmark programs while excluding the cost of simulation -> exposes the compilation and infrastructure overhead
• Can also act as an instruction tracer / program printer

Possible Drawbacks: It is a surprise.

Related GitHub Issues:

[sc-72829] [sc-72830]

[erick-xanadu]

Hi @AmintorDusko , everything I've seen looks good. Whenever you feel ready to open it for review, just change it so that it is not in draft mode. Cheers!

[github-actions]

Hello. You may have forgotten to update the changelog!
Please edit doc/releases/changelog-dev.md on your branch 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 97.93%. Comparing base (6126963) to head (7b71301).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1179      +/-   ##
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- Coverage   97.93%   97.93%   -0.01%     
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  Files          77       77              
  Lines       10921    10918       -3     
  Branches      971      971              
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- Hits        10695    10692       -3     
  Misses        179      179              
  Partials       47       47              

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[dime10]

Thanks Amintor, I'll give conditional approval here pending resolution of the last comments.