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:

• [x] Custom route generation
  • https://github.com/uselagoon/lagoon/pull/3133
• [x] Autogenerated route generation
  • https://github.com/uselagoon/lagoon/pull/3141
• [x] Backup schedule/retention configuration & generation
  • https://github.com/uselagoon/build-deploy-tool/pull/54
  • https://github.com/uselagoon/build-deploy-tool/pull/125
• [ ] DBaaS service type checks and configuration
  • https://github.com/uselagoon/build-deploy-tool/pull/115
• [ ] Fastly secret creation
• [ ] Creating services (possible https://github.com/compose-spec/compose-go)
• [x] Building and pushing images, secrets, arguments
• [x] Copy of images from external registries (configure imagecache)
• [ ] Service rollouts & status monitoring
• [x] Volumes, Deployments
• [x] Cronjobs
• [x] pre/post-rollout tasks and conditionals
• [ ] Insights generation

Scripts:

• [ ] Git checkout & merge
• [ ] Promote

Additional functionality to be added to the build-tool

• [x] AlternativeNames to add to custom routes (#25)
• [x] Support additional registries, proxies
• [ ] Add resource limits and custom metrics
• [x] k8up v2 support
• [ ] improved polysite support

[shreddedbacon]

https://github.com/uselagoon/build-deploy-tool/issues/227 is another thing we should build into here, rather than try back port into the bash