[HOLD] Add an example custom kernel defining a custom syscall

[fridrik01]

DO NOT MERGE

This is PR is part of a pluggable syscall tutorial which we will share later and contains the final source code after following the tutorial.

Context
This PR replaces the current Fendermint FVM kernel with a new custom kernel which implements a new syscall and shows how you can use this syscall from a new custom actor which we load at genesis.

More specifically, this PR:

• Implement a new `CustomKernelImpl FVM kernel which extends the default kernel.
• Implement a new syscall called my_custom_syscall on the new kernel
• Switches out the DefaultKernel used in Fendermint with this new custom kernel
• Creates a new customsyscall actor which demonstrates calling the new syscall
• Updates fendermint to load this new actor at genesis and run it at each epoch

Test plan
You can test this new kernel/syscall/actor by running one of our integration tests.

First you need to have docker installed, then compile a local docker image with this PR checked out:

cd fendermint
make docker-build

After the fendermint docker image has been built, you can run one of the integration tests

cd fendermint/testing/smoke-test
# creates the docker containers
cargo make setup
# runs the integration test
cargo make test

View fendermint logs and see the output generated by calling the customsyscall actor in each epoch:

docker ps
CONTAINER ID   IMAGE                       COMMAND
8da423d8bb1e   fendermint:latest           "fendermint --networ…"
...

View the docker logs:

docker logs 8da423d8bb1e
...
customsyscall actor returned: 21

You can now run cargo make teardown to stop the container.

[fridrik01]

This PR can be closed as its for documentation purposes