Improve setProtocol request transform types

This is useful because it means downstream we don't end up with
ws|wss|http|https which isn't compatible with either step's options.

Author: pimterry

src/rules/passthrough-handling-definitions.ts

@@ -127,9 +127,6 @@ export interface PassThroughStepConnectionOptions {
  */
 export interface PassThroughInitialTransforms {
 
-    // Made more specific in subclass overrides
-    setProtocol?: 'http' | 'https' | 'ws' | 'wss';
-
     /**
      * Replace the request host with a single fixed value, effectively forwarding
      * all requests to a different hostname.

src/rules/passthrough-handling.ts

@@ -236,7 +236,7 @@ function deriveUrlLinkedHeader(
 }
 
 export function applyDestinationTransforms(
-    transform: PassThroughInitialTransforms,
+    transform: PassThroughInitialTransforms & { setProtocol?: string },
     { isH2Downstream, rawHeaders, port, protocol, hostname, pathname, query }: {
         isH2Downstream: boolean,
         rawHeaders: RawHeaders,