Add get Directory endpoint

NEW FEATURE

Description of Feature

API endpoint to query a directory.

Description of Work Flow

API endpoint returns the contents of the directory. Files are returned as a []FileInfo and the directories' siaPaths are returned in a []string.

Testing Added

Unit testing was added for DirList(). The siatest TestRenter/TestDirectories was updated to use the new API endpoint.

Modules and Packages Impacted

• renter
• siadir
• node

Example of Feature if applicable (ie Screenshot)