Integrate SSMProblems & GeneralisedFilters with Turing.jl

[mhauru]

TODO

Edit this issue with more details about what needs to be done

[yebai]

We can likely use to_distribution(m::SSMProblem) to support linear Gaussian state-space models. For more general SSMs, we can look into to_sampleable(m::SSMProblem), which samples the latent states using a sampler in GeneralisedFilters—which can be viewed as a "submodel" but has its inference outsourced to GenerasedFilters instead of relying on MCMC on the tracing data structure VarInfo.