Provide utilities to create 'fake' requests

[pgayvallet]

Some plugins (reporting for starters) needs to fake the user authentication to impersonate them during some deferred work.

Reporting for example encrypt and store an incoming request's authentication headers to be able to have the reporting job impersonate the user when accessing SO or other ES endpoints.

kibana/x-pack/legacy/plugins/reporting/export_types/common/execute_job/get_custom_logo.ts

Lines 22 to 42 in ad356f5

	const fakeRequest: any = {
	headers: conditionalHeaders.headers,
	// This is used by the spaces SavedObjectClientWrapper to determine the existing space.
	// We use the basePath from the saved job, which we'll have post spaces being implemented;
	// or we use the server base path, which uses the default space
	getBasePath: () => job.basePath || serverBasePath,
	path: '/',
	route: { settings: {} },
	url: {
	href: '/',
	},
	raw: {
	req: {
	url: '/',
	},
	},
	};
	const savedObjects = server.savedObjects;
	const savedObjectsClient = savedObjects.getScopedSavedObjectsClient(fakeRequest);

In legacy platform, they forge the request themselves. Technically, they still could do the same in NP, However this seems fragile and relies on the underlying implementation.

Should we ease this process by providing utility methods (static on KibanaRequest or a proper API on the http service for example)?

[elasticmachine]

Pinging @elastic/kibana-platform (Team:Platform)

[pgayvallet]

Superseeded by #39430