[Python] Support named argument syntax for passing kernel parameters to `sample` and `observe` APIs

Required prerequisites

• Make sure you've read the documentation. Your issue may be addressed there.
• Search the issue tracker to verify that this hasn't already been reported. +1 or comment there if it has.
• If possible, make a PR with a failing test to give us a starting point to work on!

Describe the bug

This runs fine:

import cudaq
from cudaq import spin

n_qubits = 2

kernel, weights = cudaq.make_kernel(list)
qubits = kernel.qalloc(n_qubits)

kernel.rx(weights[0], qubits[0])
kernel.rx(weights[1], qubits[1])

h = spin.z(0)

weights = [0,1]

exp_val = cudaq.observe(kernel, h, weights).expectation_z()

But this does not:

import cudaq
from cudaq import spin


n_qubits = 2

kernel, weights = cudaq.make_kernel(list)
qubits = kernel.qalloc(n_qubits)

kernel.rx(weights[0], qubits[0])
kernel.rx(weights[1], qubits[1])

h = spin.z(0)

params = [0,1]

exp_val = cudaq.observe(kernel, h, weights = params).expectation_z()

The error it throws is also incorrect:

TypeError: observe(): incompatible function arguments. The following argument types are supported:
    1. (kernel: _pycudaq.Kernel, spin_operator: cudaq::spin_op, *args, shots_count: int = -1) -> cudaq::observe_result

Invoked with: <_pycudaq.Kernel object at 0x7f4719066030>, <_pycudaq.SpinOperator object at 0x7f4718a1dd30>; kwargs: weights=[0, 1]

Can we enable the assignment of weights in the function call?

Thanks

Steps to reproduce the bug

NA

Expected behavior

NA

Is this a regression? If it is, put the last known working version (or commit) here.

Not a regression

Environment

NA

Suggestions

No response

Start by reproducing the supplied positional and keyword-argument examples for the Python observe API, then check the corresponding sample API behavior. Done means named kernel-parameter arguments work for both APIs and invalid calls report an accurate supported signature.