Add possibility to override ephemeralStorageSizeInGiB from declarative pipeline

This PR allows to use ephemeralStorageSizeInGiB in declarative pipeline as per example

 agent {
        ecs {
            inheritFrom 'example-agent'
            cpu 1024
            memory 2048
            ephemeralStorageSizeInGiB 50
            image 'jenkins/inbound-agent:latest'
            }
        }

Testing done

This PR solves issue https://github.com/jenkinsci/amazon-ecs-plugin/issues/337

### 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

jenkinsci / amazon-ecs-plugin

Add possibility to override ephemeralStorageSizeInGiB from declarative pipeline #336

Testing done