Add support for renaming kernelspecs on the fly.

[ojarjur]

This change adds support for kernelspec managers to be configured with renaming patterns that they will apply to the kernelspecs they serve.

That, in turn, will be used a future change to allow multiple kernel spec managers to be used simultaneously without their kernel spec names colliding.

This functionality is provided using a mixin that can be inherited by a subclass of KernelSpecManager in order to add this renaming support.

Additionally, we provide canned subclasses of both KernelSpecManager and GatewayKernelSpecManager with this new renaming feature built into them.

To use the new renaming feature with a remote Kernel Gateway, the user would add the following snippet to their Jupyter config:

c.ServerApp.kernel_spec_manager_class = 'jupyter_server.gateway.managers.GatewayRenamingKernelSpecManager'

... meanwhile, an example of using the renaming functionality with local kernelspecs, can be achieved by the user adding the following snippet to their config:

c.ServerApp.kernel_spec_manager_class = 'jupyter_server.services.kernelspecs.renaming.RenamingKernelSpecManager'