Add support for containerd-config-file to bootstrap script

[suket22]

Issue #, if available:
This resolves #830 and #848
This PR continues the work from @codablock in #790

Description of changes:
Adds a new parameter to the bootstrap script --containerd-config-file that completely overrides the config to be used with containerd.
Note -

• We will continue to do a sed replace on SANDBOX_IMAGE. This is useful in many contexts, including running in cn-north-1, cn-northwest-1 where you might want to pull the pause container from ECR due to network connectivity.
• We will use the entire contents of the config file - in-line with how it works for --docker-config-json.

Testing

• I created an AMI with these changes by running make 1.21
• I launched a worker node using dockerd and containerd and made sure they join the cluster (no regressions)
• I tested the parameter and made sure the node joined and the config was applied.

To replicate my results, create some custom userData first -

> cat user-data-crconfig.txt
#!/bin/bash -xe

cat <<EOF > /etc/eks/custom-containerd-config.toml
version = 2
root = "/var/lib/containerd"
state = "/run/containerd"

[grpc]
address = "/run/containerd/containerd.sock"

[plugins."io.containerd.grpc.v1.cri".containerd]
default_runtime_name = "runc"

[plugins."io.containerd.grpc.v1.cri"]
sandbox_image = "SANDBOX_IMAGE"
max_container_log_line_size = 10000

[plugins."io.containerd.grpc.v1.cri".containerd.runtimes.runc]
runtime_type = "io.containerd.runc.v2"

[plugins."io.containerd.grpc.v1.cri".cni]
bin_dir = "/opt/cni/bin"
conf_dir = "/etc/cni/net.d"
EOF

/etc/eks/bootstrap.sh 'karp-cluster' \
--container-runtime containerd \
--containerd-config-file /etc/eks/custom-containerd-config.toml \
--kubelet-extra-args '--node-labels=test=foo'

and then launch an instance and see if it joins your cluster

> aws ec2 run-instances \
    --image-id ami-xxx \
    --instance-type t3.large \
    --count 1 \
    --subnet-id subnet-xxx \
    --security-group-ids sg-xxx \
    --iam-instance-profile Name=xx \
    --tag-specifications 'ResourceType=instance,Tags=[{Key=kubernetes.io/cluster/my-cluster,Value=owned}]' \
    --user-data file://user-data-crconfig.txt

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

[mmerkes]

LGTM