feat: support fetching skills from S3

[supreme-gg-gg]

This targets 0.10.x release. An example:

apiVersion: kagent.dev/v1alpha2
kind: Agent
metadata:
  name: s3-skills-agent
  namespace: kagent
spec:
  skills:
    s3Refs:
      - uri: s3://kagent-skills-bucket/team-a/kebab-maker // S3 folder
        name: kebab-maker
      - uri: s3://kagent-skills-bucket/bundles/ops.zip // A single archive in S3
         region: us-east-1
    initContainer:
      env:
        - name: AWS_ACCESS_KEY_ID
          valueFrom:
            secretKeyRef:
              name: aws-creds
              key: AWS_ACCESS_KEY_ID
        - name: AWS_SECRET_ACCESS_KEY
          valueFrom:
            secretKeyRef:
              name: aws-creds
              key: AWS_SECRET_ACCESS_KEY
        - name: AWS_SESSION_TOKEN
          valueFrom:
            secretKeyRef:
              name: aws-creds
              key: AWS_SESSION_TOKEN
        - name: AWS_REGION
          value: us-west-2
  type: Declarative
  declarative:
    systemMessage: You are a helpful assistant with skills.
    modelConfig: default-model-config
    tools: []

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