feat(members): add commands to manage project members

• Please check this box if this contribution uses AI-generated content (including content generated by GitLab Duo features) as outlined in the GitLab DCO & CLA. As a benefit of being a GitLab Community Contributor, you receive complimentary access to GitLab Duo.

Description

Two main commands:

• glab repo members add - Add members to a project
• glab repo members remove - Remove members from a project

Core Functionality:

• Add Command: Allows adding users by username or user ID with configurable access levels (guest, reporter, developer, maintainer, owner) and optional expiration dates
• Remove Command: Enables removing project members by username or user ID
• Access Level Support: Implements all GitLab permission levels with clear numeric mappings
• Flexible User Input: Supports both username and user ID for member identification

Related Issues

Resolves ##7299 (closed)

How has this been tested?

• Extensive unit tests for both add and remove commands
• HTTP mocking for API interactions
• Test data files for various scenarios
• Edge case coverage (invalid inputs, user not found, etc.)

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation
• Chore (Related to CI or Packaging to platforms)
• Test gap

Closes #7299 (closed)