fix(http): don't encode values that are allowed in query (#9651)

This implements a new class, QueryEncoder, that provides
methods for encoding keys and values of query parameter.
The encoder encodes with encodeURIComponent, and then
decodes a whitelist of allowed characters back to their
unencoded form.

BREAKING CHANGE:

The changes to Http's URLSearchParams serialization now 
prevent encoding of these characters inside query parameters
which were previously converted to percent-encoded values:

@ : $ , ; + ; ? /

The default encoding behavior can be overridden by extending
QueryEncoder, as documented in the URLSearchParams service.

Fixes #9348

tools/public_api_guard/http/index.d.ts

@@ -100,6 +100,12 @@ export declare abstract class JSONPConnection implements Connection {
     abstract finished(data?: any): void;
 }
 
+/** @experimental */
+export declare class QueryEncoder {
+    encodeKey(k: string): string;
+    encodeValue(v: string): string;
+}
+
 /** @experimental */
 export declare enum ReadyState {
     Unsent = 0,
@@ -209,7 +215,7 @@ export declare enum ResponseType {
 export declare class URLSearchParams {
     paramsMap: Map<string, string[]>;
     rawParams: string;
-    constructor(rawParams?: string);
+    constructor(rawParams?: string, queryEncoder?: QueryEncoder);
     append(param: string, val: string): void;
     appendAll(searchParams: URLSearchParams): void;
     clone(): URLSearchParams;