feat: add nexus_sdk extensions support

[brech1]

Is this resolving a feature or a bug?

Feature

Are there existing issue(s) that this PR would close?

• Closes Support extensions in nexus-sdk #589

If this PR is not minimal (it could be split into multiple PRs), please explain why the issues are best resolved together.

This PR is minimal.

Describe your changes.

Add extensions support to nexus-sdk. Users can now use extensions without bypassing the SDK:

use nexus_sdk::stwo::seq::{Stwo, ExtensionComponent};

let prover = Stwo::new(&elf)?.with_extensions(ExtensionComponent::keccak_extensions());
let (view, proof) = prover.prove_with_input(&input, &())?;
proof.verify_with_extensions(ExtensionComponent::keccak_extensions(), &view)?;

Changes:

• sdk: Add Stwo::with_extensions(), Proof::verify_with_extensions(), re-export ExtensionComponent
• prover/core: Expose prove_with_extensions(), verify_with_extensions(), ExtensionComponent

[github-actions]

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[brech1]

I have read the CLA Document and I hereby sign the CLA

[slumber]

Thank you for the PR.

Keccak precompile is experimental and should not be exposed via SDK yet.