chore: establish and enforce a convention for import paths

[sergiught]

Description

The main focus of this PR is to establish and enforce a convention for grouping import paths. This is easily achieved by introducing an extra check within the golangci-lint config that will:

• Move all imports into a single declaration
• Groups stdlib imports
• Groups 3rd party pkg imports
• Groups current project imports

Why?

Grouping import paths is important for several reasons:

• Improved readability
• Makes the code more predictable
• Easier to identify and manage dependencies
• Reduce cognitive load

References

• Similar to chore: establish and enforce a convention for import paths openfga#1253

Review Checklist

• I have clicked on "allow edits by maintainers".
• I have added documentation for new/changed functionality in this PR or in a PR to openfga.dev [Provide a link to any relevant PRs in the references section above]
• The correct base branch is being used, if not main
• I have added tests to validate that the change in functionality is working as expected