Feature request: network quality estimator support

*Title*: *Feature request: Network quality estimator support*

*Description*:
>For many applications, it is important to know the network quality of each client.
eg, when you watch a video on youtube, it displays different quality (1080p, 720p, 360p...), if your network quality is good, then 1080p, if weak, then 360p. This feature depends on network quality estimator. Now on **client side**, there are some implementations, see https://chromium.googlesource.com/chromium/src/+/master/net/nqe.

>I'd like to see network quality estimator support in envoy, then, the application **server side** can get network quality of each client and decides which quality of the resources(videos, images) to dispatch for the given request, It will be great for the developers of the applications.

>Thanks for your time to take a consideration!

[optional *Relevant Links*:]
>Any extra documentation required to understand the issue.
Please open this URL by Chrome: https://googlechrome.github.io/samples/network-information/index.html
Example output:
```
type: undefined
downlink: 2.3Mb/s
rtt: 100ms
downlinkMax: undefinedMb/s
effectiveType: 4g
saveData: false
```