Implement `specify_fct()`

[jonthegeek]

This function will be used to define the specification for a factor column. It's the first step in the hrmn workflow: defining the target state.

For the MVP, this function should accept a levels argument, which is a character vector of the allowed levels for the factor.

Example Usage:

# Defines a specification for a factor that must contain "active" or "inactive".
spec <- specify_fct(levels = c("active", "inactive"))