Implement the AdvancedVI interface

[sethaxen]

Even though Pathfinder is intended for initializing MCMC, it is a variational method, so we should implement the AdvancedVI.jl interface.

Currently AdvancedVI is missing an API function we would need (see TuringLang/AdvancedVI.jl#32). Also, it seems to be in the middle of a rewrite (see TuringLang/AdvancedVI.jl#25), so it might be worth waiting on this until the rewrite is finished.

[sethaxen]

While the AdvancedVI interface seems now complete, the main issue now is that AdvancedVI expects us to be able to have a step function, which would here correspond to a new optimization step followed by generation of a variational approximation and estimate of ELBO. The problem now is Optimization doesn't have any interface for iterative optimization; it's only solve API is through solve/solve!, which loops over all iterations until termination. Examples are https://github.com/SciML/Optimization.jl/blob/7a286a4392313dd9192f4e55882986fafa66808d/lib/OptimizationOptimJL/src/OptimizationOptimJL.jl#L209 and https://github.com/SciML/Optimization.jl/blob/7a286a4392313dd9192f4e55882986fafa66808d/lib/OptimizationLBFGSB/src/OptimizationLBFGSB.jl#L165

It's not quite implementing the full AdvancedVI API, but we could just overload {AdvancedVI.optimize](https://turinglang.org/AdvancedVI.jl/stable/general/#AdvancedVI.optimize) in an extension instead of using AdvancedVI's own optimization implementation. However, I don't really see what the benefit would be.