Add support for opengl external textures

[m4gr3d]

This PR makes external textures available to Godot shaders.

External textures require the GL_OES_EGL_image_external (GLES2) and GL_OES_EGL_image_external_essl3 (GLES3) extensions, so the custom_defines api is expended so that it can be used by users to include the extension when the shader is compiled.

Use cases
This can be used to embed within Godot games/apps texture(s) that can be painted by the target platform:

• playing video for all types supported by the target platform (e.g: mp4 video playback in Godot on an Android device)
• Embedding and displaying webviews provided by the target platform
• Embedding and displaying views and UIs from the target platform
• Supporting cameras and ARCore on Android
  ...

[akien-mga]

OK to merge after the 3.2.1 release.

As dicussed on #35306, this feature won't be added to the master branch for now as it depends on reimplementing GLES2 support and figure out how to use this (if relevant) with Vulkan. We'll do this later on once the master branch is closer to feature complete.

[akien-mga]

Thanks!