[feature] Use Jenkins proxy settings for Secrets Manager communication

[presPetkov]

Link to issue: https://github.com/jenkinsci/aws-secrets-manager-credentials-provider-plugin/issues/305

Jenkins url/manage/pluginManager/advanced gives the user the option to specify Http Proxy Configuration.

This configuration is not used by the aws-secrets-manager-credentials-provider-plugin.

With this pull request I propose that the plugin uses the proxy configuration if it is provided.

Testing done

Configuration

AWS credentials are not listed

The error in the logs

After installing the local build the AWS creds are listed

Testing with the proxy not set

### Submitter checklist
- [x] Make sure you are opening from a **topic/feature/bugfix branch** (right side) and not your main branch!
- [x] Ensure that the pull request title represents the desired changelog entry
- [x] Please describe what you did
- [x] Link to relevant issues in GitHub or Jira
- [x] Link to relevant pull requests, esp. upstream and downstream changes
- [x] Ensure you have provided tests - that demonstrates feature works or fixes the issue

[chriskilding]

Thanks for your update. I'm augmenting your PR with a feature that lets people set the AWS ClientConfiguration directly. This is so that they can use different proxy settings specifically for this plugin, if they don't want it to use the Jenkins system-wide proxy configuration. Will be pushing a couple more commits as I refine this

[presPetkov]

Let me know when you are finished and I will take the plugin from the build in the checks and run some tests. Thanks

[chriskilding]

@presPetkov alright, first draft complete.

The rough outline of how the feature will work now is:

• When no proxy or client configuration is set up, the plugin uses the default AWS client configuration.
• When the Jenkins proxy is configured, the plugin will use that proxy config.
• When the AWS Client Configuration is set on the plugin, the plugin will use that config. This will take precedence over the system-wide Jenkins proxy configuration (if that is present). This allows people to use a different proxy config just for Secrets Manager, if they should need to.

Give it a spin and let me know how it goes :)