feat: make upstream timeouts configurable per-remote

Keep the dial/TLS/response-header timeouts as defaults, but allow each
remote to override them via upstream_dial_timeout / upstream_tls_timeout /
upstream_response_header_timeout (seconds; 0 = default). Clients are cached
by their timeout set so remotes sharing a configuration also share a
connection pool.

Refs #67

internal/proxy/engine.go

@@ -154,7 +154,7 @@ func (e *Engine) fetchFromUpstream(ctx context.Context, remote models.Remote, pa
 		}
 	}
 
-	resp, err := upstreamClient.Do(req)
+	resp, err := clientForRemote(remote).Do(req)
 	if err != nil {
 		return nil, &UpstreamError{Err: err}
 	}
@@ -170,7 +170,7 @@ func (e *Engine) fetchFromUpstream(ctx context.Context, remote models.Remote, pa
 					req2.Header.Set("Accept", accept)
 				}
 			}
-			resp, err = upstreamClient.Do(req2)
+			resp, err = clientForRemote(remote).Do(req2)
 			if err != nil {
 				return nil, &UpstreamError{Err: err}
 			}
@@ -302,7 +302,7 @@ func (e *Engine) checkUpstream(ctx context.Context, remote models.Remote, path,
 		}
 	}
 
-	resp, err := upstreamClient.Do(req)
+	resp, err := clientForRemote(remote).Do(req)
 	if err != nil {
 		return false, &UpstreamError{Err: err}
 	}
@@ -392,7 +392,7 @@ func fetchBearerToken(ctx context.Context, wwwAuth string, remote models.Remote)
 		req.SetBasicAuth(remote.Username, remote.Password)
 	}
 
-	resp, err := upstreamClient.Do(req)
+	resp, err := clientForRemote(remote).Do(req)
 	if err != nil {
 		return "", err
 	}