Add update compose command

[olblak]

Fix #1622
Doc updatecli/website#1173

Requirements:

• Allow to share/reuse Updatecli manifest using any OCI registry #1605

This pullrquest add the support for a new configuration that allows to "compose" multiple policies.
A compose file looks like

update-compose.yaml

# Policies defines a list policies to enforce when running 
# `updatecli compose apply`
# `updatecli compose diff`
# `updatelci compose show`
#
configs:
  default:
    config:
      - updatecli.d
    values: 
      - values.yaml
    secrets:
      - secrets.yaml

policies: 
  - policy: ghcr.io/olblak/policies/updatecli:latest
  
    # The following policies are just examples and won't work as is
  - policy: ghcr.io/olblak/policies/golang:v2.1.1
  - policy: ghcr.io/olblak/policies/readme:v0.1.3

This file can be used using one of the three following commands

• updatecli compose diff : similar to updatecli diff but for multiple policies at once
• updatecli compose apply: similar to updatecli apply but for multiple policies at once
• updatecli compose show : similar to updatecli show but for multiple policies at once

Test

To test this pull request, you can run the following commands:

go build -o bin/updatecli
./bin/updatecli compose diff -f update-compose.yaml

Additional Information

Tradeoff

Potential improvement

• Generating a jsonschema and publish on the jsonschema store

[olblak]

Remark, in the initial idea, I was considering to use a map for the policies but after additional thoughts I think an array is better