feat: enhance MCPServer env var configuration with EnvVarCfg type

[drewhemm]

This PR attempts to address #98

It maintains backwards compatibility for specifying env vars using simple strings (value), but also adds the ability to specify object values, which unlock the ability to use valueFrom, to source individual env vars from secrets, configmaps and the Downward API.

Example:

---
apiVersion: kagent.dev/v1alpha1
kind: MCPServer
metadata:
  name: my-mcp-server
  namespace: my-mcp-namespace
spec:
  deployment:
    env:
      FOO: bar
      VAR_FROM_SECRET:
        valueFrom:
          secretKeyRef:
            name: secret-name
            key: secret-key
      MY_CPU_REQUEST:
        valueFrom:
          resourceFieldRef:
            containerName: test-container
            resource: requests.cpu