Reduce bandwidth consumption of the integration

[jdejaegh]

Until now, the integration would download all the data (forecast, radar images, pollen) every 7 minutes without client caching. This pull request brings the following improvements:

• Only download current radar frame and localization layer when the user brings the radar in the viewport
• Download the full sequence of images only if the user shows the animation
• Locally cache the responses and re-use cached data based on the ETag value set the by remote server

The data is still refreshed every 7 minutes with local caching when responses come with the ETag header. Only the radar animation is downloaded on-demand. Short-term rain forecast shown on the radar is still downloaded every 7 minutes (this is part of the forecast file anyway).