Add a "features" option to `enw_add_metaobs_features()`

[seabbs]

enw_add_metaobs_features() is currently used to add features such as holidays, a day of the week variable and numeric variables for the week and month. It is used to generate metadata for modelling in enw_preprocess_data() as well as in enw_expectation() if a renewal process is used with latent reporting delays (as this is initialised with additional data up to the maximum delay of the latent reporting delay).

For the second use case, it would be helpfuil to be able to pass a features data.frame` to be bound to the new extended dataset as otherwise complex features may be hard to use in models. This would also be helpful once forecasting has been added as this would also need to extend this dataset as well as several others.