feat: implement opacity to control visibility of subtitles (#3583)

* feat: implement opacity to control visibility of subtitles

implemented per discussion on https://github.com/react-native-video/react-native-video/issues/3579

updated docs and linked onTextTrackDataChanged to the subtitle style to clarify intent on how to control visibility.

* chore: update type so that we use a union of 0 1 vs any number

* chore: run ktlint to get rid of white spaces

* feat: add ability to have range of numbers for opacity; while, 0 will still not render the subtitles.

added util function for safeGetFloat

updated types

* feat: add ability to suppress subtitles with opacity 0

add data structure for subtitle styles for extensibility

* chore: run yarn check-ios

* chore: update documentation to clarify differences between android and ios

* Update android/src/main/java/com/brentvatne/exoplayer/ExoPlayerView.java

Co-authored-by: Olivier Bouillet <62574056+freeboub@users.noreply.github.com>

---------

Co-authored-by: Olivier Bouillet <62574056+freeboub@users.noreply.github.com>

ios/Video/DataStructures/SubtitleStyle.swift

@@ -0,0 +1,17 @@
+struct SubtitleStyle {
+    // Extend with more style properties as needed.
+    private(set) var opacity: CGFloat
+
+    enum SubtitleStyleKeys {
+        static let opacity = "opacity"
+    }
+
+    init(opacity: CGFloat = 1) {
+        self.opacity = opacity
+    }
+
+    static func parse(from dictionary: [String: Any]?) -> SubtitleStyle {
+        let opacity = dictionary?[SubtitleStyleKeys.opacity] as? CGFloat ?? 1
+        return SubtitleStyle(opacity: opacity)
+    }
+}