feat: allow ssh-credentials to be set in agent and plugin

[42atomys]

Description

This pull request introduces the capability to configure ssh credentials directly in the Buildkite agent and plugin settings. This feature enhances security and flexibility by allowing users to set up git access credentials, improving the management and use of private repositories in CI/CD workflows.

This pull request put gitFromEnv as deprecated because this is not relevante to the usage (currently this field is used to set a ssh credentials and can be also used as envFrom override). The gitFromEnv still working for backward compatibility but as the project are not stable yet, what is the vision about deprecated lifecycle ?

⚠️ DEPRECATED ⚠️

The method for setting git credentials in Buildkite agents stack and k8s plugins is changing. Please update your configurations to use the new method introduced in PR #248 for enhanced security and flexibility.

# DEPRECATED WAY
gitEnvFrom:
- secretRef: { name: agent-stack-k8s }
# REPLACEMENT
ssh-credentials-secret: agent-stack-k8s