HVP for everyone

[gdalle]

Core

• Define gradient_and_hessian_vector_product for every second order backend (except symbolic ones), mutation-compatible or not
• Better supports_mutation and support_hessian utilities, checking correct result (that way FiniteDiff over FiniteDiff fails)

Tests

• Add second-order tests for more backends, especially SecondOrder
• Add correctness, stability and allocation tests for gradient_and_hessian_vector_product
• Better scenario display in test set

Extensions

• Enzyme: Make sure to convert dx everywhere before building Duplicated(x, dx)

[gdalle]

Enzyme is crapping all over the test logs: EnzymeAD/Enzyme.jl#1354