[Files] Use server-side authc.getCurrentUser from core.security (#186922

)

Part of #186574

Background: This PR serves as an example of a plugin migrating away from
depending on the Security plugin, which is a high priority effort for
the last release before 9.0. The Files plugin uses the
`authc.getCurrentUser` method to attribute the current user to files
that are created in the system.

### Checklist

Delete any items that are not applicable to this PR.

- [ ] [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

src/plugins/files/server/routes/file_kind/create.ts

@@ -31,12 +31,12 @@ export type Endpoint<M = unknown> = CreateRouteDefinition<
   FilesClient['create']
 >;
 
-export const handler: CreateHandler<Endpoint> = async ({ fileKind, files }, req, res) => {
-  const { fileService, security } = await files;
+export const handler: CreateHandler<Endpoint> = async ({ core, fileKind, files }, req, res) => {
+  const [{ security }, { fileService }] = await Promise.all([core, files]);
   const {
     body: { name, alt, meta, mimeType },
   } = req;
-  const user = security?.authc.getCurrentUser(req);
+  const user = security.authc.getCurrentUser();
   const file = await fileService.asCurrentUser().create({
     fileKind,
     name,

src/plugins/files/server/routes/types.ts

@@ -17,14 +17,12 @@ import type {
   ResponseError,
   RouteMethod,
 } from '@kbn/core/server';
-import type { SecurityPluginStart } from '@kbn/security-plugin/server';
 import type { FileServiceStart } from '../file_service';
 import { Counters } from '../usage';
 import { AnyEndpoint } from './api_routes';
 
 export interface FilesRequestHandlerContext extends RequestHandlerContext {
   files: Promise<{
-    security?: SecurityPluginStart;
     fileService: {
       asCurrentUser: () => FileServiceStart;
       asInternalUser: () => FileServiceStart;