upgrade qml.qsvt

[KetpuntoG]

• Add changelog

This PR upgrades qml.qsvt based on this PRD . To check how the documentation is displayed -> [here]

The main changes are:

• old qml.qsvt only worked with qml.BlockEncode. With the newqml.qsvt the user can choose the specific block encoding.
• new function also accept Hamiltonians as input instead of just matrices.
• In old qml.qsvt the user had to pass the appropriate angles to apply a polynomial. Now the user passes the polynomial directly

These changes aim to make qml.qsvt, the friendly (but less customizable) version to use QSVT.
Old functionality is moved to qml.qsvt_legacy.

[sc-72651] [sc-72650]

[github-actions]

Hello. You may have forgotten to update the changelog!
Please edit doc/releases/changelog-dev.md with:

• A one-to-two sentence description of the change. You may include a small working example for new features.
• A link back to this PR.
• Your name (or GitHub username) in the contributors section.

[Jaybsoni]

Looks good, left some suggestions. Let me know once they are addressed and I am happy to review again!

Also missing change-log entry 😉

[KetpuntoG]

Thanks for the review @Jaybsoni 🙌
I answered your questions and included your suggestions
I will add the changelog at the end when we are clear when we are going to merge it to avoid early conflicts

[DSGuala]

Thanks!
Left a few comments about errors I ran into and potential doc/error message improvements.

[AntonNI8]

Looks good for the most part, just needs:
qsvt_legacy:
-deprecation warning, suggestion to use qsvt instead.
-update deprecation docs accordingly.
qsvt:
-Raise a warning when qsvt is called without the new keyword arguments.
-In the warning, state the (new) function signature, and ask users to provide poly rather than angles, and to use the keyword arguments to silence the warning.
-Refer them to qsvt_legacy for access to the previous functionality.

[Jaybsoni]

Looks good to me, just a few minor suggestions.

Regarding Diego's comment about block-encoding, I am confused as to why it's causing issues. Let's chat offline to discuss the technical problem. Conditionally approving based on the result of that discussion.

[KetpuntoG]

Based on the last discussion with Jay and Diego:

• BlockEncode will be the new default for matrices
• There is a more explanatory error in the cases where FABLE does not work

[DSGuala]

Looks nearly good to go. Approved pending a few comments 👍