PauliSum.to_list() remove signed Pauli expression

[sesmart]

Describe the bug

Creating a PauliSum preserves the sign in the coefficient as well as in the PauliString. For conversion to a list format, we should remove the sign, as the CircuitBinding list format expects a simple string, i.e. "X", not "+X". Maybe add a flag?

To reproduce

test= PauliSum([(0.5, "XX"), (0.3, "YY")])
test.to_list()
# returns 
# [(0.5, "+XX"), (0.3, "+YY")]

Expected behavior
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System information
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• Amazon Braket Python SDK version:
• Amazon Braket Python Schemas version:
• Amazon Braket Python Default Simulator version:
• Python version:

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