[Prototype] Caching in QiskitDevice

[antalszava]

This PR prototypes the logic of adding caching capabilities to QiskitDevice.

[codecov]

Codecov Report

Merging #166 (7feb9fd) into master (0998b85) will decrease coverage by 4.80%.
The diff coverage is 30.43%.

@@             Coverage Diff             @@
##            master     #166      +/-   ##
===========================================
- Coverage   100.00%   95.19%   -4.81%     
===========================================
  Files            7        7              
  Lines          314      333      +19     
===========================================
+ Hits           314      317       +3     
- Misses           0       16      +16     

Impacted Files	Coverage Δ
pennylane_qiskit/qiskit_device.py	91.57% <30.43%> (-8.43%)	⬇️

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[antalszava]

Closing this as device caching is being deprecated (see here PennyLaneAI/pennylane#2154).