Proxy support for ProcessCredentialsProvider

### Describe the feature

Allow passing of environment variables to ProcessCredentialsProvider so it's possible to support for example proxy with `aws_signing_helper`

### Use Case

Example Use-Case:
using IamAuthAnywere with the `aws_signing_helper` inside a locked down environment it is neccisary to set the environment variable `http_proxy` / `https_proxy`.

### Proposed Solution

extend the `ProcessCredentialsProvider.Builder` to accept environments and forward it to `ProcessBuilder`.

### Other Information

it's possible to expose these variables as global, but it's already cumbersome to set all the different jvm proxy props, having them as additional global variables would make it even more confusing.

### Acknowledgements

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

### AWS Java SDK version used

2.20.161

### JDK version used

openjdk 11.0.19 2023-04-18 LTS

### Operating System and version

-