feat(server): init worker app with multi-module workspaces

[HideBa]

What

• Create worker dir
• Introduce multi-module workspaces so that we can have multiple modules even in the same workspace
• Add Dockerfile
• Add a simple server which returns "hello world"

Why

• Since Go1.18 supports the multi-module workspaces feature which allows us to have multiple modules under a single workspace though we couldn't do that and were forced to have a hacky way, I've introduced that to take the full advantage of mono-repo.

Point of Review

• If there is any problem with directory structure, Dockerfile, etc

Things you don't need to review

• The implementation of a simple server since it's a temporary development
• GitHub actions workflow file will be implemented later

[rot1024]

Yes it's time to use go workspace!