[Fleet][EPM] If proxy url has username & password, add to Node's Agent options (#86807)

## Summary

A user received a `407` response when using a `registryProxyUrl` format like `http://user:pass@host:port`

I believe the issue is we're not including the [`auth` property as described in this issue](https://github.com/TooTallNate/node-https-proxy-agent/issues/12#issuecomment-216098644). 

Add tests to ensure it only adds `auth` if username & password are given.

### Checklist
Delete any items that are not applicable to this PR.
- [x] [Unit or functional tests](https://www.elastic.co/guide/en/kibana/master/development-tests.html) were updated or added to match the most common scenarios

```
  getProxyAgentOptions
    auth property
      present & correct if given username and password
        ✓ proxy url is http (1 ms)
        ✓ proxy url is https
      missing if not given username and password
        ✓ proxy url is http
        ✓ proxy url is https
```

x-pack/plugins/fleet/server/services/epm/registry/proxy.test.ts

@@ -67,4 +67,65 @@ describe('getProxyAgentOptions', () => {
       rejectUnauthorized: undefined,
     });
   });
+  describe('auth property', () => {
+    describe('present & correct if given username and password', () => {
+      test('proxy url is http', () => {
+        const httpProxyWithAuth = {
+          proxyUrl: 'http://user:pass@example.com:8080/p/a/t/h',
+          targetUrl: 'https://epr.elastic.co/',
+        };
+        expect(getProxyAgentOptions(httpProxyWithAuth)).toEqual({
+          auth: 'user:pass',
+          headers: { Host: 'epr.elastic.co' },
+          host: 'example.com',
+          port: 8080,
+          protocol: 'http:',
+          rejectUnauthorized: undefined,
+        });
+      });
+      test('proxy url is https', () => {
+        const httpsProxyWithAuth = {
+          proxyUrl: 'https://user:pass@example.com:8080/p/a/t/h',
+          targetUrl: 'https://epr.elastic.co/',
+        };
+        expect(getProxyAgentOptions(httpsProxyWithAuth)).toEqual({
+          auth: 'user:pass',
+          headers: { Host: 'epr.elastic.co' },
+          host: 'example.com',
+          port: 8080,
+          protocol: 'https:',
+          rejectUnauthorized: undefined,
+        });
+      });
+    });
+
+    describe('missing if not given username and password', () => {
+      test('proxy url is http', () => {
+        const httpProxyWithout = {
+          proxyUrl: 'http://example.com:8080/p/a/t/h',
+          targetUrl: 'https://epr.elastic.co/',
+        };
+        expect(getProxyAgentOptions(httpProxyWithout)).toEqual({
+          headers: { Host: 'epr.elastic.co' },
+          host: 'example.com',
+          port: 8080,
+          protocol: 'http:',
+          rejectUnauthorized: undefined,
+        });
+      });
+      test('proxy url is https', () => {
+        const httpsProxyWithoutAuth = {
+          proxyUrl: 'https://example.com:8080/p/a/t/h',
+          targetUrl: 'https://epr.elastic.co/',
+        };
+        expect(getProxyAgentOptions(httpsProxyWithoutAuth)).toEqual({
+          headers: { Host: 'epr.elastic.co' },
+          host: 'example.com',
+          port: 8080,
+          protocol: 'https:',
+          rejectUnauthorized: undefined,
+        });
+      });
+    });
+  });
 });

x-pack/plugins/fleet/server/services/epm/registry/proxy.ts

@@ -38,11 +38,15 @@ export function getProxyAgent(options: GetProxyAgentParams): ProxyAgent {
 export function getProxyAgentOptions(options: GetProxyAgentParams): HttpsProxyAgentOptions {
   const endpointParsed = new URL(options.targetUrl);
   const proxyParsed = new URL(options.proxyUrl);
+  const authValue = proxyParsed.username
+    ? `${proxyParsed.username}:${proxyParsed.password}`
+    : undefined;
 
   return {
     host: proxyParsed.hostname,
     port: Number(proxyParsed.port),
     protocol: proxyParsed.protocol,
+    auth: authValue,
     // The headers to send
     headers: options.proxyHeaders || {
       // the proxied URL's host is put in the header instead of the server's actual host