Add kube inject command

[versilis]

This adds a new command akita kube inject that can be used to manually inject Kuberentes YAML configuration files. Along with injecting deployments, it also can generate a secret to a file or stdout with the use of the --secret flag.

This PR depends on #207 for its injection functionality.

Example usages:

# Print injected resources to stdout
akita kube inject -f in.yml

# Print secret and injected resources to stdout. (combining all using `---`)
akita kube inject -s -f in.yml

# Output injected resource to file, and also generate and merge any required secrets
akita kube inject -s -f in.yml -o out.yml

# Output injected resources and generated secrets to separate files
akita kube inject -s="secret.yml" -f in.yml -o out.yml

# Applying via pipe
akita kube inject -f in.yml | kubectl -f -

# Applying via file
akita kube inject -f in.yml -o out.yml && kubectl apply -f out.yml

Example Output (w/merged Secrets):

---
apiVersion: v1
kind: Secret
metadata:
  name: akita-secrets
  namespace: default
type: Opaque
data:
  akita-api-key: ****
  akita-api-secret: ***
---
apiVersion: v1
kind: Secret
metadata:
  name: akita-secrets
  namespace: ns1
type: Opaque
data:
  akita-api-key: ***
  akita-api-secret: ***
---
apiVersion: apps/v1
kind: Deployment
metadata:
  creationTimestamp: null
  name: test-deploy
  namespace: default
spec:
  replicas: 1
  selector:
    matchLabels:
      app: test-pod
  strategy: {}
  template:
    metadata:
      creationTimestamp: null
      labels:
        app: test-pod
    spec:
      containers:
      - image: ghcr.io/wzshiming/echoserver/echoserver:v0.0.1
        name: test-container
        resources: {}
      - args:
        - apidump
        - --project
        - docker-extension-testing
        env:
        - name: AKITA_API_KEY_ID
          valueFrom:
            secretKeyRef:
              key: akita-api-key
              name: akita-secrets
        - name: AKITA_API_KEY_SECRET
          valueFrom:
            secretKeyRef:
              key: akita-api-secret
              name: akita-secrets
        image: akitasoftware/cli:latest
        lifecycle:
          preStop:
            exec:
              command:
              - /bin/sh
              - -c
              - AKITA_PID=$(pgrep akita) && kill -2 $AKITA_PID && tail -f /proc/$AKITA_PID/fd/1
        name: akita
        resources: {}
        securityContext:
          capabilities:
            add:
            - NET_RAW
status: {}
---
apiVersion: apps/v1
kind: Deployment
metadata:
  creationTimestamp: null
  name: patch-demo
  namespace: ns1
spec:
  replicas: 2
  selector:
    matchLabels:
      app: nginx
  strategy: {}
  template:
    metadata:
      creationTimestamp: null
      labels:
        app: nginx
    spec:
      containers:
      - image: nginx
        name: patch-demo-ctr
        resources: {}
      - args:
        - apidump
        - --project
        - docker-extension-testing
        env:
        - name: AKITA_API_KEY_ID
          valueFrom:
            secretKeyRef:
              key: akita-api-key
              name: akita-secrets
        - name: AKITA_API_KEY_SECRET
          valueFrom:
            secretKeyRef:
              key: akita-api-secret
              name: akita-secrets
        image: akitasoftware/cli:latest
        lifecycle:
          preStop:
            exec:
              command:
              - /bin/sh
              - -c
              - AKITA_PID=$(pgrep akita) && kill -2 $AKITA_PID && tail -f /proc/$AKITA_PID/fd/1
        name: akita
        resources: {}
        securityContext:
          capabilities:
            add:
            - NET_RAW
      tolerations:
      - effect: NoSchedule
        key: dedicated
        value: test-team
status: {}

[liujed]

@versilis, the change set here includes everything from #207, so if I were to review this as is, I'd also end up reviewing #207, which is currently assigned to Mark. Would you mind either making the reviewer of both PRs the same, or re-opening this as a PR against #207?

[liujed]

This looks reasonable. Before I approve, I have some questions about the intended output for both the secrets file and the Kubernetes YAML. The part I am least sure about is the output of createSidecar, but I assume all this will be (or has been) tested with a Kubernetes instance (or, at the very least, compared with a known working Kubernetes YAML with Akita installed).