Forward any registry cache-control header for files (elastic#83680)

closes elastic#83631 

### Problem
Assets are served with a `cache-control` header that prevents any caching
<img src="https://user-images.githubusercontent.com/640/99534379-517d2300-2975-11eb-8c05-4fb3f127c52b.png"/>

### Root cause
Likely from this code https://github.com/elastic/kibana/blob/2a365ff6329544465227e61141ded6fba8bb2c80/src/core/server/http/http_tools.ts#L40-L43

Also based on these tests, it seems this is default/expected behavior

https://github.com/elastic/kibana/blob/b3eefb97da8e712789b5c5d2eeae65c886ed8f64/src/core/server/http/integration_tests/router.test.ts#L510-L520

### Proposed solution
Set the header via the response handler as shown in this test:
https://github.com/elastic/kibana/blob/b3eefb97da8e712789b5c5d2eeae65c886ed8f64/src/core/server/http/integration_tests/router.test.ts#L522-L536

### This PR
If this registry response contains a `cache-control` header, that value is included in the EPM response as well

In `master`, which points to `epr-snapshot`
<img width="742" alt="Screen Shot 2020-11-18 at 12 33 47 PM" src="https://user-images.githubusercontent.com/57655/99568352-4fc75580-299d-11eb-962f-6ff28fa9510d.png">
which matches https://epr-snapshot.elastic.co/package/apache/0.2.6/img/logo_apache.svg

or using `epr.elastic.co`, 
<img width="781" alt="Screen Shot 2020-11-18 at 12 31 56 PM" src="https://user-images.githubusercontent.com/57655/99568350-4fc75580-299d-11eb-966e-f3489c13edb5.png">
which matches https://epr.elastic.co/package/apache/0.2.6/img/logo_apache.svg

Author: John Schulz

x-pack/plugins/fleet/server/routes/epm/handlers.ts

@@ -4,7 +4,7 @@
  * you may not use this file except in compliance with the Elastic License.
  */
 import { TypeOf } from '@kbn/config-schema';
-import { RequestHandler, CustomHttpResponseOptions } from 'src/core/server';
+import { RequestHandler, ResponseHeaders, KnownHeaders } from 'src/core/server';
 import {
   GetInfoResponse,
   InstallPackageResponse,
@@ -103,15 +103,21 @@ export const getFileHandler: RequestHandler<TypeOf<typeof GetFileRequestSchema.p
   try {
     const { pkgName, pkgVersion, filePath } = request.params;
     const registryResponse = await getFile(`/package/${pkgName}/${pkgVersion}/${filePath}`);
-    const contentType = registryResponse.headers.get('Content-Type');
-    const customResponseObj: CustomHttpResponseOptions<typeof registryResponse.body> = {
+
+    const headersToProxy: KnownHeaders[] = ['content-type', 'cache-control'];
+    const proxiedHeaders = headersToProxy.reduce((headers, knownHeader) => {
+      const value = registryResponse.headers.get(knownHeader);
+      if (value !== null) {
+        headers[knownHeader] = value;
+      }
+      return headers;
+    }, {} as ResponseHeaders);
+
+    return response.custom({
       body: registryResponse.body,
       statusCode: registryResponse.status,
-    };
-    if (contentType !== null) {
-      customResponseObj.headers = { 'Content-Type': contentType };
-    }
-    return response.custom(customResponseObj);
+      headers: proxiedHeaders,
+    });
   } catch (error) {
     return defaultIngestErrorHandler({ error, response });
   }