Implement `specify_df()`

[jonthegeek]

This function will be the container for defining a full data frame specification. It will act as the top-level specification constructor for the most common use case.

MVP Behavior:

• It should accept named arguments in ....
• Each argument will be a call to another specify_* function (for the MVP, specify_fct).
• It will return a structured list or object that represents the data frame specification.

Example Usage:

df_spec <- specify_df(
  status = specify_fct(levels = c("active", "inactive"))
)