Provide additional context for externally provided file metadata formatters

#1867236: Allow other modules to define their own file_metadata display formatters provides the actual metadata value being rendered, but not the context under which it's being rendered ... which means that the formatter doesn't know key information such as the current $node->nid value (essential for creating a comment jumplink!).

We should pass the full $context array to file_metadata_table_celldata(), so that it's available to pass on to the formatter.