remove static lifetime for name str parameter requirement for constant getter

Description

Hey!
I noticed get_constant_from_var_name has a different signature from other hint_utils helper functions, namely that it has a bound of static lifetime on the name parameter. I noticed it mainly comes from the error signature HintError::MissingConstant.

I changed the signature of MissingConstant to match other errors: from Box<'static &str> to Box using the same util you use around the hint error codebase: var_name.to_string().into_boxed_str().

In doing this, one no longer needs to have 'static strings for constant getting -> this will allow me in next PRs to have dynamic constant name getting, i.e., in garaga there are constant limbs of Prime P of the BN Curve.
And depending on N_LIMBS, we'll get P_i.

Let me know if this works!