add a project.lock_status endpoint to the service

[Panaetius]

Currently, long running operations like cache.migrate lock the project to other requests, raising an exception of a lock couldn't be acquired withing 15 seconds.

We should add a project.lock_status endpoint that can let clients know if a project is currently (write-)locked, returning a boolean.

The code for this could be something like

if "project_id" in self.context:
    project = self.cache.get_project(self.user, self.context["project_id"])
elif "git_url" in self.context:
    project = Project.get(
        (Project.user_id == self.user_data["user_id"]) & (Project.git_url == self.context["git_url"])
    )
else:
    raise Exception()

locked = True

try:
    with project.read_lock(timeout=0):
        locked=False
except (portalocker.LockException, portalocker.AlreadyLocked):
    locked = True

return locked

and of course without using the RenkuOperationMixin, similar to how renku.service.views.jobs doesn't use the mixin.

[m-alisafaee]

Discuss with UI to make sure that the API here is what they want.