Provide AD-based defaults for GaussNewton strategy, and default for ContinuousMultivariateLogPdf

I think my use case may be general/common enough to bake in as the default for projecting Continuous(Uni)MultivariateLogPdf functions using the Bonnet/GaussNewton strategies.

The following is the code I use to project my ContinuousMultivariateLogPdf onto Gaussian:

# using previously defined my_pdf <: ContinuousMultivariateLogPdf
function my_logpdf!(out, x)
    out[1] = my_pdf.logpdf(x)
    return out
end
function my_grad!(out, x)
    out .= ForwardDiff.gradient(my_pdf.logpdf, x)
    return out
end
function my_hess!(out, x)
    out .= ForwardDiff.hessian(my_pdf.logpdf, x)
    return out
end

inplace = ExponentialFamilyProjection.InplaceLogpdfGradHess(my_logpdf!, my_grad!, my_hess!)
params = ProjectionParameters(
    tolerance = 1e-8,
    strategy = ExponentialFamilyProjection.GaussNewton(nsamples = 1), # deterministic
)
prj = ProjectedTo(MvNormalMeanCovariance, D; parameters = params)
result = project_to(prj, inplace) # <--- note inplace

So, it seems to me that what is feasible and good would be to allow the following to be automatically equivalent to the above by default, without providing the three mutation-functions:

# using previously defined my_pdf <: ContinuousMultivariateLogPdf
params = ProjectionParameters(
    tolerance = 1e-8,
    strategy = ExponentialFamilyProjection.GaussNewton(nsamples = 1), # deterministic
)
prj = ProjectedTo(MvNormalMeanCovariance, D; parameters = params)
result = project_to(prj, my_pdf) # <--- note my_pdf is here now, instead of inplace

And so we are saying, okay, if you provide just a pdf with this strategy, we assume you intend AD-based grad/hess.

And sure, if we want a different AD-library (e.g. DifferentiationInterface.jl), we bake that in instead

Good? :)

Start by reading the ProjectedTo, project_to, ProjectionParameters, and GaussNewton entry points mentioned in the example, then trace how ContinuousMultivariateLogPdf inputs are handled. Determine how the default AD-based gradient and Hessian behavior should interact with the strategy choice. Done means the PDF-only example works equivalently to the explicit InplaceLogpdfGradHess form, with the default behavior verified.