Migrate existing legacy build functionality from bash to golang

[tobybellwood]

The existing build stages that are currently in Bash, that need migrating across to golang here:

Build Steps:

• Custom route generation
  • feat: use build tool to generate ingress templates lagoon#3133
• Autogenerated route generation
  • feat: use build tool to generate autogen routes lagoon#3141
• Backup schedule/retention configuration & generation
  • Add backup config generation #54
  • Change: use build-deploy-tool for backup schedule generation #125
• DBaaS service type checks and configuration
  • Change: use build-tool for generating dbaas consumers in builds #115
• Fastly secret creation
• Creating services (possible https://github.com/compose-spec/compose-go)
• Building and pushing images, secrets, arguments
• Copy of images from external registries (configure imagecache)
• Service rollouts & status monitoring
• Volumes, Deployments
• Cronjobs
• pre/post-rollout tasks and conditionals
• Insights generation

Scripts:

• Git checkout & merge
• Promote

Additional functionality to be added to the build-tool

• AlternativeNames to add to custom routes (Feature: add alternative names and wildcard support to lagoon.yml #25)
• Support additional registries, proxies
• Add resource limits and custom metrics
• k8up v2 support
• improved polysite support

[shreddedbacon]

#227 is another thing we should build into here, rather than try back port into the bash