Implementation of signal_state for newly added BlockEncoding instances.

[charlesyuan314]

See #1094.

This method should be implemented for all BlockEncoding instances once the new interface around PrepareOracle has stabilized.

[mpharrigan]

Can you add more detail about what's required here

[charlesyuan314]

We need to do the following:

• Create a quantum interface for state preparation. It should expose essentially the same signature as BlackBoxPrepare, which should be made an instance.
• Implement signal_state for all BlockEncoding instances to return an instance of that quantum interface. In most cases (including linear combination, product, Chebyshev polynomial) this will be PrepareIdentity.
• In composite block encodings, add checks that the signal state of the constituents is permitted. In most cases this check will require that the signal state be PrepareIdentity.

[charlesyuan314]

One challenge here is that we have been increasingly committing to the PrepareOracle abstract class as the interface of state preparation. This class is not a quantum interface; it generates its signature dynamically just like the old BlockEncoding interface did.

I haven't yet come to a conclusion on how to add/change the Prepare interface in a way that isn't highly confusing/redundant with the existing PrepareOracle and BlackBoxPrepare.