[android] Handle user login

[Kuchteq]

Android

Currently, the groups which the user sees/can access isn't limited to their account. We want to change that by implementing the login functionality on the android client.

• [x] Make necessary forms/text fields to handle passing the login data. Those include the instance url, username and password.
• [x] Prepare the server request and response model.
• [x] Add the interface function for logging in inside ApiService (/repositories/ApiClient.kt), use the request and response model.
• [ ] Make the LoginViewModel handle the input from the text fields and use it to make the call to the API. The design of it is available at https://www.figma.com/design/lzUOvguPdSF8103bGHBnDX/Split?node-id=0-1&t=YZP5CRpQmUWGEy0Q-1
• [ ] The user first makes a call to the specified instance at /ping to detect if the instance even works. If we get a 200, we allow the user to proceed to the log in and input password. If the request response is any other than 200, we will display that something hasn't succeeded.
• [ ] Store the returned token inside encrypted sharedPreferences (https://developer.android.com/reference/androidx/security/crypto/EncryptedSharedPreferences)
• [ ] Extract username and userID from the JWT token and also store it inside encrypted sharedPreferences

Server

• [x] Make a GET request handler at /ping that just returns the 200 code.