automate DTP's injection of auto-converters so extensions don't have to do this

[jzacsh]

for context, see the TODO on lines of extensions/data-transfer/portability-data-transfer-microsoft/src/main/java/org/datatransferproject/transfer/microsoft/MicrosoftTransferExtension.java in https://github.com/google/data-transfer-project/pull/1062/files inlined below:

  // TODO(jzacsh,wmorland) don't keep adding here - just have the converters invoked automatically
  // when Media isn't supported on one or the other side of this equation; this is just a WIP
  // prototype to show the concept of converters at play.
  private static final ImmutableList<String> SUPPORTED_IMPORT_SERVICES =
      ImmutableList.of(CALENDAR, CONTACTS, PHOTOS, MEDIA);