[Maintenance Windows] Create go module for MaintenanceWindowPolicy resolver

[lindnerby]

Description

In lifecycle-manager we create a new go module /pkg/maintwindow containing the logic for the window resolver as seen in internal.
Adding tests and notifying orchestration-operator team that the pkg can be used by them and shared.

Reasons

We want to offer the code for resolving MaintenanceWindowPolicy and ResolvedWindow to be shared.

Acceptance Criteria

• New go.mod in /pkg/maintwindow containing resolver logic
• Logic is unit tested
• Ping orchestration-operator team (= SRE) that they can use and import the dependency to their project

Testing approach

• Unit testing

Related issue

• Support maintenance window for module upgrades kyma#18611

[ruanxin]

Notes

In Go projects, the pkg folder is typically used to store packages intended for use by other projects. If we place this resolver under pkg, it implies that other projects would depend directly on the lifecycle manager project, which is not our intention. Instead, we want this library to be a standalone Go module.

For reference, you can find numerous examples of how the pkg folder is used in other projects here:
https://github.com/golang-standards/project-layout/tree/master/pkg
Notably, there seems to be no concern regarding designing Go modules under pkg in these examples.

This also highlights another issue:

• the existing pkg folder should be moved to internal since none of the current packages are designed for sharing purposes. Relocate package in pkg folder to internal since none of the current packages are designed for sharing purposes #2143

I propose we consider an alternative folder structure or use the api module instead. Personally, I prefer using the api module for sharing this resolver code, as it aligns well with its intended purpose.

[Tomasz-Smelcerz-SAP]

I decided to go with a dedicated directory: /maintenancewindows to host the new module