Example of S3 upload using STS

### Before opening, please confirm:

- [X] I have [searched for duplicate or closed issues](https://github.com/aws-amplify/amplify-android/issues?q=is%3Aissue+) and [discussions](https://github.com/aws-amplify/amplify-android/discussions).

### Language and Async Model

Kotlin - Coroutines

### Amplify Categories

Authentication, Storage

### Gradle script dependencies

### Environment information

### Please include any relevant guides or documentation you're referencing

_No response_

### Describe the feature request

Hello, team.
we need general guidance for the simplest example in the world.

### we have:
1. PHP API backend. it implements login-functionality
2. android mobile application (and swift application also)
3. AWS infrastructure

### we need to implement:
upload to S3 from mobile apps

### basic flow:
a) user logins using PHP API backend on mobile app. receives auth `TOKEN` back
b) user chooses file and presses button Upload

so to implement upload we should use for example Amplify.
**BUT WITHOUT ANY COMPLEX WRAPPERS** like Cognito, Sign-In with Apple, Sign-In with Amazon, Sign-In with Facebook and etc. we don't want to have user database on AWS or sync it there from PHP backend.

as far as i feel it is possible to make using STS. saying mobile app receives from PHP API backend temporary STS-token and will use it to call AWS. BUT i don't see how this STS can be injected into Amplify?

### questions:
1. what is **SEVERAL** possible approaches to implement this on simplest manner using our architecture?
2. how to inject STS token into Amplify Storage and other services?

thanks!

### Initialization steps (if applicable)

_No response_

### Code Snippet

_No response_

### amplifyconfiguration.json

_No response_

### GraphQL Schema

### Additional information and screenshots

_No response_

The issue mentions an Android app using Kotlin coroutines, a PHP API, AWS STS, and S3, but names no repository files, tests, or entry points. Start by reviewing the Amplify Android authentication and Storage documentation for custom credentials, then determine which supported architecture and configuration examples would answer the requested flow.

Written by the indexing model from the issue text.