[FEATURE] Add ExtraContainers to SharedDeploymentSpec

[renato0307]

📋 Prerequisites

• I have searched the existing issues to avoid creating a duplicate
• By submitting this issue, I agree to follow the Code of Conduct

📝 Feature Summary

Add support for extraContainers in SharedDeploymentSpec to allow injecting sidecar containers alongside the main kagent agent container.

❓ Problem Statement / Motivation

• SharedDeploymentSpec currently supports volumes, volumeMounts, and env for customizing agent pods, but there is no way to inject additional containers alongside the primary kagent container.
• Platform teams running kagent in multi-tenant Kubernetes environments need to inject sidecar containers (e.g. token-injecting proxies, log shippers, security agents) into agent pods without forking kagent or using a MutatingAdmissionWebhook.
• A MutatingWebhook adds operational complexity and a new component to manage. The cleanest solution is a first-class field in the Agent CRD that passes through to the generated Deployment.

💡 Proposed Solution

Add an extraContainers field to SharedDeploymentSpec:

// ExtraContainers is a list of additional containers to run alongside the main agent container.
// +optional
ExtraContainers []corev1.Container `json:"extraContainers,omitempty"`

The translator (adk_api_translator.go) appends them after the primary container:

Containers: append([]corev1.Container{{ /* kagent container */ }}, dep.ExtraContainers...),

The change is minimal (~20 lines across agent_types.go, deployments.go, and adk_api_translator.go), fully backward-compatible (optional field, defaults to empty), and follows the exact same pattern as the existing volumes and volumeMounts fields.

🔄 Alternatives Considered

• MutatingAdmissionWebhook — works but adds a separate webhook component with TLS management overhead.
• Extending proxy.url — kagent's proxy mechanism handles MCP/A2A traffic but cannot inject sidecars or cover all outbound call paths.

🎯 Affected Service(s)

Controller Service

📚 Additional Context

A reference implementation is available at renato0307/kagent@feat/extra-containers, including the CRD change, translator update, codegen output, and a golden test case.

🙋 Are you willing to contribute?

• I am willing to submit a PR for this feature