Git support for smartcards

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As smartcard support from Git was missing, we used a workaround in https://gitlab.com/gitlab-org/gitlab-ee/issues/6990 to check if the user has an active browser session where smartcard was used for login. This was acceptable as a first iteration but we eventually want git itself to support smartcard authentication and use it in GitLab.

Some examples of what will be required in git and curl: https://gitlab.com/gitlab-org/gitlab-ee/issues/6990#note_180753538

/cc @jeremy @lmcandrew @cdmaurer13

Edited by 🤖 GitLab Bot 🤖