accept dict for header styling (#5905)

I found that it's harder to style a header with the list using index,
they are to be applied by header name. Otherwise, we might end up using
a different header for coloring, or worse throw some error because the
header name and style don't match.
Row does have it's property of index, so it makes sense there.

tests/unit/ui/test_table.py

@@ -119,6 +119,7 @@ def test_rich_border(capsys: CaptureFixture[str]):
     "extra_opts",
     [
         {"header_styles": [{"style": Style(bold=True)}]},
+        {"header_styles": {"first": {"style": Style(bold=True)}}},
         {"row_styles": [{"style": Style(bold=True)}]},
     ],
 )