feat: Kraus implementation for density matrix MCM

[speller26]

Uses Kraus formalism to implement mid-circuit measurements in braket_dm; this allows shots=0 simulation of dynamic circuits.

As well, instead of rerunning a simulation for each branch, this implementation merges intermediate ensembles during the course of a simulation. This takes more space than the branching algorithm, but the intermediate merging prevents the full exponential space explosion from branching.

Issue #, if available:
#368

Description of changes:

Testing done:

Merge Checklist

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (6055367) to head (b0b1707).

Additional details and impacted files

@@            Coverage Diff             @@
##              main      #382    +/-   ##
==========================================
  Coverage   100.00%   100.00%            
==========================================
  Files           49        50     +1     
  Lines         4919      5161   +242     
  Branches       569       603    +34     
==========================================
+ Hits          4919      5161   +242     

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