Add support for custom adjacency context in masked autoregressive transform

This PR implements [#64](https://github.com/probabilists/zuko/issues/64), adding support for user-defined structural constraints in the context features of MaskedAutoregressiveTransform. Allows specifying an adjacency matrix of shape (features, features + context). The right-most columns correspond to context-related constraints. Adds assertions to verify shape validity. Includes a test to ensure correct behavior. No additional assumptions are made about the structure of the context adjacency beyond its shape. Feel free to suggest any changes or additional constraints to enforce.