Implement automatic subsampling for `DynamicPPL` models

[Red-Portal]

It has been agreed on Slack to implement subsampling for Turing models using a syntax similar to that discussed here in AdvancedVI. This should be handled by adding a Turing extension that implements the routine subsample.

[Red-Portal]

This has now been implemented. But tests have not yet been added. As soon as tests are put in place, I'll release v0.7.