feat: add support for oci tasks

[willswire]

Description

This is an experimental change!!

Adds initial support for including OCI artifacts as tasks.

Try it out!

1. Build with go build
2. Run a docker registry locally with: docker run -d -p 5000:5000 --name registry registry:2.7
3. Create a tasks file to push remotely:

# hello.yaml
tasks:
  - name: world
    actions:
      - cmd: echo "Hello from an OCI artifact task!"
      - cmd: echo "This task is being executed from a container registry."

  - name: inputs
    inputs:
      name:
        description: "Your name"
        default: "Friend"
      message:
        description: "Custom message"
        default: "Welcome to Maru OCI tasks!"
    actions:
      - cmd: echo "Hello, ${INPUT_NAME}!"
      - cmd: echo "${INPUT_MESSAGE}"

4. Push with ./maru-runner push --insecure hello.yaml oci://localhost:5000/hello:0.0.1
5. Create a tasks file to include the remote artifact:

# tasks.yaml
includes:
  - hello: oci://localhost:5000/hello:0.0.1

tasks:
  - name: default
    actions:
      - cmd: echo "Testing OCI artifact integration"
      - task: hello:world

  - name: with-args
    actions:
      - task: hello:inputs
        with:
          name: "OCI Test User"
          message: "This is a test of the OCI tasks feature!"

6. Run with ./maru-runner run --insecure

The --insecure flag is only required for insecure registries. Push/pull to Docker Hub or GHCR to test out secure pushes and includes.

Related Issue

Supersedes #204

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Other (security config, docs update, etc)

Checklist before merging

• Test, docs, adr added or updated as needed
• Contributor Guide Steps followed