add ability to pass a list of groups

[amenocal]

would resolve #23

This pull request mainly focuses on enhancing the GitLab-Stats CLI tool by introducing a new feature that allows users to gather metrics from specific GitLab groups.

Enhancements to the GitLab-Stats CLI tool:

• README.md: Updated the instructions on how to run the tool and the usage section to include information about the new groups flag. This flag allows users to specify a comma-separated list of groups from which to gather metrics. Also added an upgrade section.

New functions and modifications:

• api/groups/groups.go: Added new functions GetGroupsByName and GetGroupsProjects to fetch GitLab groups by name and to get projects within these groups respectively.
• api/projects/projects.go: Added a new function GetProject to fetch a GitLab project along with its statistics.
• cmd/root.go: Modified the init and getGitlabStats functions to handle the new groups flag and to fetch GitLab groups and projects based on the provided group names. Also added a new function GetGitLabGroupsProjects to fetch projects from the specified groups. [1] [2]

New file:

• internal/groups.go: Created a new file containing the GetGroupsFromNames function which fetches GitLab groups based on the provided group names.