Allow to pass `extra_env` down to terminado to set `PS1` variable in terminals

[krassowski]

Problem

We need a way to set the terminal prompt (e.g. with PS1 environment variable) for reproducible snapshots as in jupyterlab/jupyterlab#17656. This may be also desirable for real deployments. While terminado classes accept both shell_command and extra_env, the latter is not currently supported in jupyter-server traitlet definition:

jupyter_server/jupyter_server/serverapp.py

Lines 1491 to 1495 in 04dd3e7

	terminado_settings = Dict(
	Union([List(), Unicode()]),
	config=True,
	help=_i18n('Supply overrides for terminado. Currently only supports "shell_command".'),
	)

Proposed Solution

Use per-key traitlets to accept shell_command as an union of string/list and extra_env as a dict with string keys.

Additional context

Need to add a test around here:

jupyter_server/tests/test_terminal.py

Lines 278 to 297 in 04dd3e7

	@pytest.mark.parametrize(
	"terminado_settings,expected_shell,min_traitlets",
	[
	("shell_command=\"['/path/to/shell', '-l']\"", ["/path/to/shell", "-l"], "5.4"),
	('shell_command="/string/path/to/shell -l"', ["/string/path/to/shell", "-l"], "5.1"),
	],
	)
	def test_shell_command_override(
	terminado_settings, expected_shell, min_traitlets, jp_configurable_serverapp
	):
	pytest.importorskip("traitlets", minversion=min_traitlets)
	argv = shlex.split(f"--ServerApp.terminado_settings={terminado_settings}")
	app = jp_configurable_serverapp(argv=argv)
	if os.name == "nt":
	assert app.web_app.settings["terminal_manager"].shell_command in (
	expected_shell,
	" ".join(expected_shell),
	)
	else:
	assert app.web_app.settings["terminal_manager"].shell_command == expected_shell