Document new saas param configs: `options`, `multiselect` and `unpack`

[adamsachs]

Docs Update Description

#1253 adds a few new options to parameters in SaaS configs:

A connector_param, can now have:

• an options field, which defines a set (enumeration) of values that are allowed for the given param. Providing a value that's outside of this set will result in a configuration error.
• a multiselect field, which can only be used if options is defined on the same param. multiselect indicates that multiple values from the defined options enumeration may be used on any given parameterization, provided as an array. it defaults to False.

A request's param_value has the new field:

• unpack, a boolean which defaults to False indicating that the values must be unpacked from a list/array before being used to generate the requests. This means that a list of n elements will result in n distinct requests for the given traversal. The primary use case driving this option is one-to-many relationships, e.g. a given user record is associated with n mailing lists, and the users collection has a multi-value list_ids field pointing to the IDs of these lists. Each request to the mailing_lists endpoint takes only a single list_id as a param value, but the reference from the users collection will resolve to an array of n values for any given user record. Upon traversing from the users endpoint to the mailing_lists endpoint, fidesops must unpack the array of n values in the list_id field into n requests to the mailing_lists endpoint, each with a single list_id from the array.

Additional context

See updates to saas_example_config.yml and saas_example_dataset.yml from the PR for examples of the new functionality being configured