Add list_project_members MCP tool

Giannis Kepasrequested to merge
gitlab-community/gitlab-org/gitlab:597676-list-project-members-mcp into master

What does this MR do and why?

Adds list_project_members MCP tool for agents while following the implementation guidelines and plan described in the related issue.

Changelog: added
EE: true

Related to #597676

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