This change adds a CardTheme and tests for it.

Golden is here:
https://github.com/flutter/goldens/blob/ec26eeebb44068599c100d3f0c2db7102b047737/packages/flutter/test/material/card_theme.custom_shape.png

From commit:
https://github.com/flutter/goldens/commit/ec26eeebb44068599c100d3f0c2db7102b047737#diff-7564b206413654283ddc5cb59ecd64d4