GitHub proxy: git to prompt for password when tsh session expires

[greedy52]

related:

• GitHub proxy MVP #48762

changelog: git commands executed in terminals (now support interactive login prompts when the tsh session expires

Did not add a unit test since it's difficult to mock.

Sample log before the change:

2025-01-21T14:35:49.858-05:00 DEBU [CLIENT]    Relogin is not available in this environment error:[
ERROR REPORT:
Original Error: *errors.errorString underlying reader is not a terminal
Stack Trace:
	github.com/gravitational/teleport/lib/client/api.go:4873 github.com/gravitational/teleport/lib/client.(*TeleportClient).AskPassword
	github.com/gravitational/teleport/lib/client/api.go:4111 github.com/gravitational/teleport/lib/client.(*TeleportClient).localLogin
	github.com/gravitational/teleport/lib/client/api.go:3717 github.com/gravitational/teleport/lib/client.(*TeleportClient).getSSHLoginFunc.func2
	github.com/gravitational/teleport/lib/client/api.go:3909 github.com/gravitational/teleport/lib/client.(*TeleportClient).SSHLogin.func1
	github.com/gravitational/teleport/lib/client/api.go:3962 github.com/gravitational/teleport/lib/client.(*TeleportClient).loginWithHardwareKeyRetry
	github.com/gravitational/teleport/lib/client/api.go:3907 github.com/gravitational/teleport/lib/client.(*TeleportClient).SSHLogin
	github.com/gravitational/teleport/lib/client/api.go:3497 github.com/gravitational/teleport/lib/client.(*TeleportClient).Login
	github.com/gravitational/teleport/lib/client/api.go:675 github.com/gravitational/teleport/lib/client.RetryWithRelogin
	github.com/gravitational/teleport/tool/tsh/common/git_ssh.go:83 github.com/gravitational/teleport/tool/tsh/common.(*gitSSHCommand).run
	github.com/gravitational/teleport/tool/tsh/common/tsh.go:1656 github.com/gravitational/teleport/tool/tsh/common.Run
	github.com/gravitational/teleport/tool/tsh/common/tsh.go:644 github.com/gravitational/teleport/tool/tsh/common.Main
	github.com/gravitational/teleport/tool/tsh/main.go:26 main.main
	runtime/proc.go:272 runtime.main
	runtime/asm_arm64.s:1223 runtime.goexit
User Message: cannot perform password login without a terminal
	underlying reader is not a terminal] trace_id:a7b8b7690ed59e49842906951f2d57a3 span_id:96231e274f9c4507 client/api.go:678

After this change (tested on mac and amazon linux)

$ git fetch
Enter password for Teleport user admin:
Available MFA methods [WEBAUTHN, OTP]. Continuing with WEBAUTHN.
If you wish to perform MFA with another method, specify with flag --mfa-mode=<sso,otp>.

Tap any security key
Detected security key tap