Support TypedDataFilters in Data Fetcher

Problem/Motivation

It would be great to apply filters using the DataFetcher service. Currently the placeholder resolver service uses DataFetcher to get the data, then applies the filters, then converts the result to a string - all in one method.

In some applications of DataFetcher, you want to apply a filter but not convert to a string at the end, for example, adding two days to a datetime item.

Proposed resolution

1. Take the parseMainPlaceholderPart method and make it public on the data fetcher service
2. Add an applyFilters method to the DataFetcher service
3. Update placeholder resolver service to use the new methods

User interface changes

None

API changes

Two new public methods on the data fetcher service.

Data model changes

None