builder: add support for proxying requests to composer

We need koji-osbuild-builder to be able to connect to composer via a proxy
because koji builders in our internal deployment cannot reach
api.openshift.com directly. This commit adds a new option `proxy` to the
builder plugin config that controls whether a proxy is used to route all
requests to composer.

plugins/builder/osbuild.py

@@ -469,6 +469,16 @@ class OSBuildImage(BaseTaskHandler):
             self.client.http.verify = val
             self.logger.debug("ssl verify: %s", val)
 
+        proxy = composer.get("proxy")
+        if proxy:
+            # route both http and https requests through the proxy
+            proxies = {
+                "http": proxy,
+                "https": proxy
+            }
+            self.client.http.proxies.update(proxies)
+            self.logger.debug("proxy: %s", proxy)
+
         if "composer:oauth" in cfg:
             oa = cfg["composer:oauth"]
             client_id, client_secret = oa["client_id"], oa["client_secret"]