Enable Checkpointing

[fivegrant]

This PR exposes two actions add_checkpoint and rollback which serializes the kernel's state (variables, functions, and imported modules) and allows the kernel to be restored to the previous saved state. Currently, checkpointing is only implemented for the Python subkernel.

Changes from this PR:

• BaseCheckpointableSubkernel is the new class that contains the new functionality. Subkernels still can inherit from the original BaseSubkernel instead if we want to avoid checkpointing for a specific language.
• The context checks if the subkernel contains actions and registers them.
• connected_kernel is no longer in beaker-kernel. Instead, it is a property of the subkernel. We're encapsulating more functionality inside of the subkernel itself.
• The subkernel has been moved from beaker-kernel to the context along with execute and evaluate.

Work for Future PRs

• Integrate checkpointing into beaker-ts, beaker-vue and the development interface. The checkpointing feature is functional but is not very accessible.
• (once the above is figured out) Create a 'execute code block' tool for agent. Since state can be rolled back, we can allow the agent to run arbitrary code and enable the user to revert to a previous state before the kernel was polluted.

[fivegrant]

@mattprintz and I came up with a few TODOs

• [x] Make the connection kernel a property of the subkernel. ProxyKernelClient shouldn't be the parent class.

• [x] Move subkernel into BaseContext

• [ ] Move the following functions into BaseContext

  • [x] execute
  • [x] evaluate
  • [ ] send_preview

• [x] Rename current checkpoint to something like 'most recent checkpoint'

• [x] Use tempfile module instead of writing to /tmp

• [x] Break out checkpoint functionality into class BaseCheckpointKernel