Update qml.expval to correctly cast value type

[andrijapau]

Context:

Prior to this PR we had the expectation value being silently converted to a real type,

obs = (0.2 + 0.7j) * qml.PauliZ(0)

@qml.qnode(qml.device("default.qubit", 1))
def qnode():
    return qml.expval(obs)

>>> qnode()
0.2

Description of the Change:

This PR stops casting to a real number when calculating expval and instead casts to whatever the eigenvalue type is.

Benefits:

Code is now correct with the following behaviour,

obs = (0.2 + 0.7j) * qml.PauliZ(0)

@qml.qnode(qml.device("default.qubit", 1))
def qnode():
    return qml.expval(obs)

>>> qnode()
(0.2+0.7j)

Possible Drawbacks: None.

Related GitHub Issues: Fixes #6076

[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 99.59%. Comparing base (0ecf816) to head (e3a953c).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6939      +/-   ##
==========================================
- Coverage   99.59%   99.59%   -0.01%     
==========================================
  Files         480      480              
  Lines       45505    45504       -1     
==========================================
- Hits        45320    45319       -1     
  Misses        185      185              

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[mudit2812]

This test doesn't need to do a device execution. You can manually check that the correct expval is returned using ExpectationMP.process_state and ExpectationMP.process_samples.