[feat] Use `credentials_file` variable in config file

### Describe the feature

I want to keep user secrets separate from config.

Secrets file at custom path `/home/me/.secrets/aws/credentials`:
```ini
[default]
aws_access_key_id=foo
aws_secret_access_key=bar
```

Config file at default path `/home/me/.aws/config`:
```ini
[default]
region=us-east-1
credentials_file=/home/me/.secrets/aws/credentials
```

I assumed I can use `credentials_file` as it's in [the docs](https://github.com/aws/aws-cli/tree/v2?tab=readme-ov-file#other-configurable-variables). However that doesn't work:

> Unable to locate credentials. You can configure credentials by running "aws configure".

Please let us use `credentials_file` as a variable in the config file.

### Use Case

- To keep secrets and config separate
- To keep secrets in well-known location
- Plays nice with other tools

### Proposed Solution

_No response_

### Other Information

_No response_

### Acknowledgements

- [ ] I may be able to implement this feature request
- [ ] This feature might incur a breaking change

### CLI version used

2.18.1

### Environment details (OS name and version, etc.)

ubuntu 22.04

Start with the linked AWS CLI documentation and the configuration and secrets-file examples in the issue. Reproduce the failure using the listed paths and AWS CLI 2.18.1 on Ubuntu 22.04, then trace the configuration and credential-loading entry points. Done means a credentials_file value in /home/me/.aws/config is honored and credentials are located from the custom path.

Written by the indexing model from the issue text.