Add getUserByExternalId (#1229)

## Description

Convenience method for accessing the associated api endpoint

## Documentation

Does this require changes to the WorkOS Docs? E.g. the [API
Reference](https://workos.com/docs/reference) or code snippets need
updates.

```
[x] Yes
```

If yes, link a related docs PR and add a docs maintainer as a reviewer.
Their approval is required.

^ docs are coming but looking to get this out in a preview release ahead
of time.

Author: ericroberts

src/user-management/user-management.spec.ts

@@ -59,6 +59,29 @@ describe('UserManagement', () => {
     });
   });
 
+  describe('getUserByExternalId', () => {
+    it('sends a Get User request', async () => {
+      const externalId = 'user_external_id';
+      fetchOnce({ ...userFixture, external_id: externalId });
+
+      const user = await workos.userManagement.getUserByExternalId(externalId);
+      expect(fetchURL()).toContain(
+        `/user_management/users/external_id/${externalId}`,
+      );
+      expect(user).toMatchObject({
+        object: 'user',
+        id: 'user_01H5JQDV7R7ATEYZDEG0W5PRYS',
+        email: '[email protected]',
+        profilePictureUrl: 'https://example.com/profile_picture.jpg',
+        firstName: 'Test 01',
+        lastName: 'User',
+        emailVerified: true,
+        lastSignInAt: '2023-07-18T02:07:19.911Z',
+        externalId,
+      });
+    });
+  });
+
   describe('listUsers', () => {
     it('lists users', async () => {
       fetchOnce(listUsersFixture);

src/user-management/user-management.ts

@@ -199,6 +199,14 @@ export class UserManagement {
     return deserializeUser(data);
   }
 
+  async getUserByExternalId(externalId: string): Promise<User> {
+    const { data } = await this.workos.get<UserResponse>(
+      `/user_management/users/external_id/${externalId}`,
+    );
+
+    return deserializeUser(data);
+  }
+
   async listUsers(options?: ListUsersOptions): Promise<AutoPaginatable<User>> {
     return new AutoPaginatable(
       await fetchAndDeserialize<UserResponse, User>(