fix(SUP-42112): Advanced Caption Settings not working in iOS (#783)

issue:
in advances captions settings for ios change background color and change
font size didn't applied

solution:
add font-size to css attribute
change background to use gradient (due to known issue in ios) to css
attribute

resolve [SUP-42112](https://kaltura.atlassian.net/browse/SUP-42112)

[SUP-42112]:
https://kaltura.atlassian.net/browse/SUP-42112?atlOrigin=eyJpIjoiNWRkNTljNzYxNjVmNDY3MDlhMDU5Y2ZhYzA5YTRkZjUiLCJwIjoiZ2l0aHViLWNvbS1KU1cifQ

src/track/text-style.ts

@@ -283,7 +283,8 @@ class TextStyle {
     attributes.push('text-align: ' + this.textAlign);
     attributes.push('font-family: ' + this.fontFamily);
     attributes.push('color: ' + TextStyle.toRGBA(this.fontColor, this.fontOpacity));
-    attributes.push('background-color: ' + TextStyle.toRGBA(this.backgroundColor, this.backgroundOpacity));
+    attributes.push(`background: linear-gradient(0deg, ${TextStyle.toRGBA(this.backgroundColor, this.backgroundOpacity)}, ${TextStyle.toRGBA(this.backgroundColor, this.backgroundOpacity)})`);
+    attributes.push('font-size: ' + this.fontSize);
     attributes.push('text-shadow: ' + this.getTextShadow());
     return attributes.join('!important; ');
   }