feat: add expo plugins (#3933)

* feat: add expo plugins

* add export

* fix import

* fix bugs

* build `lib` to `CommonJS`

* restore `build.gradle`

* remove plugin tmp

* add expo plugin for ios caching

* add docs for expo plugin

* fix expo plugin export

* fix docs

android/build.gradle

@@ -15,8 +15,11 @@ buildscript {
     }
 }
 
+// This looks funny but it's necessary to keep backwards compatibility (: 
 def safeExtGet(prop) {
-    return rootProject.ext.has(prop) ? rootProject.ext.get(prop) : project.properties["RNVideo_" + prop]
+    return rootProject.ext.has(prop) ?
+            rootProject.ext.get(prop) : rootProject.ext.has("RNVideo_" + prop) ?
+            rootProject.ext.get("RNVideo_" + prop) : project.properties["RNVideo_" + prop]
 }
 
 def isNewArchitectureEnabled() {