feat: Enable load_witness functionallity within FCircuit

Since the witness is generated in the browser we need a way to store it
such that it can flow from `prove_step` into `generate_constraints` due
to the API restrictions.

folding-schemes/src/folding/nova/circuits.rs

@@ -410,6 +410,9 @@ where
 
         // get z_{i+1} from the F circuit
         let i_usize = self.i_usize.unwrap_or(0);
+
+        // If we are in the browser-frontend case. The witness is already loaded within
+        // self.F.witness. This was done at `self.prove_step()` fn.
         let z_i1 = self
             .F
             .generate_step_constraints(cs.clone(), i_usize, z_i, external_inputs)?;

folding-schemes/src/frontend/mod.rs

@@ -48,6 +48,13 @@ pub trait FCircuit<F: PrimeField>: Clone + Debug {
         z_i: Vec<FpVar<F>>,
         external_inputs: Vec<FpVar<F>>, // inputs that are not part of the state
     ) -> Result<Vec<FpVar<F>>, SynthesisError>;
+
+    /// Allows to load pre-generated witness into the FCircuit implementor.
+    /// This is only needed by the browser use cases where we have already computed our
+    /// witness there. And we need a way to load it into the FCircuit since it's already computed.
+    ///
+    /// By default this method will simply do nothing. Only in the browser FCircuit implementors this will have usage.
+    fn load_witness(&mut self, _witness: Vec<F>) {}
 }
 
 #[cfg(test)]