[KSW-Operator] Initialize Go Workspace to manage multi-module structure

Since at the moment the Kogito Serverless Workflow Operator has got 4 mdoules:

• root
• api
• container-builder
• workflowproj

We should use initialize a Go Workspace (https://go.dev/blog/get-familiar-with-workspaces) to better manage the life of this multi-module project even because there are some cross dependencies between some modules (workflowproj -> api) that need to be synced together.