Add a "indexFilterBeginsWith" string param to POST /search by meta

[reganwolfrom]

In order to create a UI to select a folder as a destination for moving a document or folder, it would be more efficient to be able to retrieve only folders in POST /search, rather than retrieving documents as well and needing to filter.

e.g.
{"query":{"meta":{"folder":""}},"responseFields":{"tags":["sysFavoritedBy","sysSharedWith","sysDeletedBy","english","marketing","path","Test","untagged"]}}

{"query":{"meta":{"folder":"Version/"}},"responseFields":{"tags":["sysFavoritedBy","sysSharedWith","sysDeletedBy","english","marketing","path","Test","untagged"]}}

[reganwolfrom]

We may want to add a filter on content-type, and add a content-type for folders