upstream ext_proc: option to wait for connection establishment

*Title*: *upstream ext-proc: option to wait for connection establishment*

*Description*:
Upstream HTTP filters are not guaranteed to execute after upstream connection establishment: https://github.com/envoyproxy/envoy/issues/10455#issuecomment-1446519546.

Upstream HTTP filters may require information about the upstream peer (such as Peer Cert Subject/IP) and application-layer data from the request, to determine if a request may be proxied or blocked, especially in DFP use cases.

Ext-Proc filters can expose an option to wait for upstream connection establishment before sending headers and attributes to the ext-proc server, to ensure that this info is available.