Add support for selecting multiple months and milestones in GitLab Wrapped

Currently, GitLab Wrapped appears to support single-period analysis.

Users need the ability to select multiple months and milestones simultaneously to create more comprehensive and flexible reports covering custom time periods and project scopes.

User Story

As a project manager/developer, I want to select multiple months and milestones so that I can generate wrapped reports for custom time periods (e.g., Q1-Q2, specific project phases) and compare metrics across different milestones within those periods.