[RFC] Create a python type or submodule analogous to `cudaq::types::GATE`

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Describe the feature

When adding noise channels in C++, a programmer passes the gate for the channel as a cudaq::types::GATE template argument. Example:

noise.add_channel<cudaq::types::x>(...)

In Python, however, we rely on strings for this gate specification

noise.add_channel('x', ...)

My suggestion is to create an enum, or a new types submodule, that mirrors the C++ version.

I believe this will make the interface a bit easier to follow, especially if you pass in an invalid gate. As is, an invalid gate will return the error

    noise.add_channel('e', [0], bit_flip_zero)
RuntimeError: Invalid quantum op for noise_model::add_channel (e).

which is a clear error message, but doesn't give a good starting point for finding acceptable gates. You would have to refer to our kernel builder documentation to find the list of gates, and not all are allowed here.

This change would look more like this:

a) A completely invalid type

    noise.add_channel(cudaq.types.E, [0], channel)
AttributeError:'cudaq.types' object has no attribute 'E'

b) valid cudaq::type that we can't add noise to. This is more likely in a scenario where the types class is re-used elsewhere and has to be extended beyond just the allowed noisy gates.

    noise.add_channel(cudaq.types.mz, [0], channel)
RuntimeError: Invalid `cudaq.types` for noise_model::add_channel (cudaq::types::mz).

Start by comparing the C++ cudaq::types::GATE interface with the Python noise.add_channel gate argument described here. Define the Python type or submodule behavior, including acceptable noisy gates and handling of invalid or unsupported types; the issue provides no file or test paths to begin with.