Introduce Git* extensions to Che-Theia

[vitaliy-guliy]

Is your enhancement related to a problem? Please describe.

Need to add following VSCode extension to Che-Theia

• git-ui https://github.com/microsoft/vscode/tree/master/extensions/git-ui
• github-authentication https://github.com/microsoft/vscode/tree/master/extensions/github-authentication
• github https://github.com/microsoft/vscode/tree/master/extensions/github

They are already published to Open VSX Registry

• https://open-vsx.org/extension/vscode/git-ui
• https://open-vsx.org/extension/vscode/github-authentication
• https://open-vsx.org/extension/vscode/github

[vinokurig]

• github-authentication plugin registers a GitHub authentication provider specific for vscode: https://github.com/microsoft/vscode/blob/1961260ef3f6993b9c57f7dadccb00f423a1c1b2/extensions/github-authentication/src/githubServer.ts#L79
  Che-Theia already has its own GitHub authentication provider: https://github.com/eclipse/che-theia/blob/f48828db6dcf691f8f2b3c887496f67c1a293a55/plugins/github-auth-plugin/src/github-auth-plugin.ts#L18
• github plugin is already proposed by Support more than one authentication session che-theia#906
• git-ui plugin only registers one command: https://github.com/microsoft/vscode/blob/ca22de4b17af4ec84394537546998bba195e84cd/extensions/git-ui/src/main.ts#L14
  and I haven't found any places where this command is used in vscode. probably this command is needed for some external plugins. I am not sure we really need this plugin.

[che-bot]

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