Add a command for validating lockfile validity/freshness

[tgolsson]

Is your feature request related to a problem? Please describe.

I would like a cheap way of verifying that my lockfile is up to date, for example for CI usage.

Describe the solution you'd like

While developing a plugin for version management of torch binaries I found this function:

pdm/src/pdm/cli/actions.py

Lines 155 to 156 in 7877f73

	def check_lockfile(project: Project, raise_not_exist: bool = True) -> str | None:
	"""Check if the lock file exists and is up to date. Return the update strategy."""

That seems like it would be very helpful if it was exposed either as a new command (pdm validate-lockfile?) or as pdm lock --check.

I can contribute whichever variant you'd prefer - hence why I'm opening this to get a feeler for preferences.

[frostming]

I'd vote for pdm lock --check(in fact, there is pdm install --check), PR welcome