Use standardized format for upstream names in HTTP/gRPC callouts

[PiotrSikora]

Right now, the upstream name used inside plugin must match configured upstream name in the host, but we don't provide any recommendations around its format, which makes this feature less portable than it could be.

As such, I suggest that we standardize on URI's scheme://authority (e.g. https://www.google.com or udp://1.2.3.4:53) as the upstream name... although that doesn't allow configuration of TLS client certificates, etc.

Note: There is an undocumented behavior in Envoy that allows passing xDS proto with cluster configuration in place of the upstream name, which breaks the security properties of the sandbox, but that's not something supported per existing specification and it shouldn't be used.

cc @kyessenov