Reusable proxies

[alex-schneider]

Labeled proxy blocks defined in definitions can be referenced with the proxy attribute in an endpoint:

server {
  api {
    endpoint "/a" {
      beta_required_permission = "pr1"
      proxy = "p"
    }
    endpoint "/a" {
      beta_required_permission = "pr2"
      proxy = "p"
    }
    endpoint "/c" {
      beta_required_permission = "pr3"
      proxy = "p"
    }
  }
}
# ...
definitions {
  proxy "p" {
    backend = "b"
    # ...
  }
  backend "b" {
    # ...
  }
}

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