REST xDS API with HTTP GET requests

*Description*:

In some cases, some of Envoy users want to use a object storage like Amazon S3 as a control plane. This is mainly for that simplicity and most of Envoy configurations are almost static in that case. For example, if one wants to use only Envoy's failure recovery features, the one can deliver that configuration to Envoy instances via RDS and CDS. That CDS response can point to a DNS name to central (server-side) load balancer and it's not so dynamic. Also, RDS can response route config with timeouts and retries and they are not updated frequently in this case.

*Considerations*:

- URL mapping
- How to encode DiscoveryRequest into HTTP GET requests
- v2 XDS has version communication mechanism (as far as I know)

I don't know yet we can have well design and implementation for this feature, so this needs comments and further investigation.

*Relevant Links*:

- Conversation on Slack: https://envoyproxy.slack.com/archives/C78M4KW76/p1537878857000100
- One use case of using object storage as a v1 CDS/RDS API: https://blog.envoyproxy.io/service-mesh-and-cookpad-ba4d5d915dbd