Add Git Credential Manager

[RayJW]

Requesting a new app ✨

• I have checked that this app does not already have a pending request and has not previously been declined.
• App Name: Git Credential Manager
• Download URL: https://github.com/GitCredentialManager/git-credential-manager/releases/latest
• Website URL: https://github.com/GitCredentialManager/git-credential-manager/
• App Summary:

Git Credential Manager (GCM) is a secure Git credential helper built on .NET that runs on Windows, macOS, and Linux.

Compared to Git's built-in credential helpers (Windows: wincred, macOS: osxkeychain, Linux: gnome-keyring/libsecret) which provides single-factor authentication support working on any HTTP-enabled Git repository, GCM provides multi-factor authentication support for Azure DevOps, Azure DevOps Server (formerly Team Foundation Server), GitHub, Bitbucket, and GitLab.

Git Credential Manager (GCM) replaces the .NET Framework-based Git Credential Manager for Windows (GCM), and the Java-based Git Credential Manager for Mac and Linux (Java GCM), providing a consistent authentication experience across all platforms.

[natanjunges]

Closed by #621.