feat: add support for webassembly for kubectl

[remyleone]

What type of PR is this?

/kind feature

What this PR does / why we need it:

This PR adds support for compiling kubectl to webassembly. With both js and wasip1.

From a product perspective, having a kubectl inside a browser to perform some actions would be quite cool and would also help for training environment :) It would allow to spawn from the browser a version of kubectl that would not require an external shell/VM to run. It would be run in the browser.

Which issue(s) this PR is related to:

#128853

Special notes for your reviewer:

This is preliminary work to ensure that the compilation step is passing. Maybe other PRs will come to fix particular behaviours that are happening in the context.

I'm not familiar about how to specify a new target only for webassembly and kubectl. Obviously it makes no sense to have kubelet and other binaries supported in this OS/Arch for the time being.

At the time being there are two issues:

1. For GOOS=js we do not have access to the filesystem and that is a problem because kubectl needs to do a syscall to open a kubeconfig on the filesystem. One possible solution would be to have kubectl reading the entire kubeconfig from an environement variable inside the webassembly. This would be distinct from today as currently kubectl only reads path and perform an open. We would need to have an environment variable such as KUBECONFIG_INLINE/KUBECONFIG_RAW.
2. For GOOS=wasip1 we do not have access to the terminal therefore all the code in moby/term would not work natively and it would need to be handle by polyfills in the browser. This would theoretically remove the need to have support for loading kubeconfig from an environment variable.

Does this PR introduce a user-facing change?

User would be able to run kubectl in a browser that support webassembly which is pretty cool and would open new usage for running commands without requiring any pre-installation regardless of the platform.

Additional documentation e.g., KEPs (Kubernetes Enhancement Proposals), usage docs, etc.:

[k8s-ci-robot]

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[k8s-ci-robot]

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[k8s-ci-robot]

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[remyleone]

Upstream PR for term moby/term#49

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: remyleone
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[k8s-ci-robot]

@remyleone: The following tests failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
pull-kubernetes-dependencies	c2c063d	link	true	/test pull-kubernetes-dependencies
pull-kubernetes-cmd	c2c063d	link	true	/test pull-kubernetes-cmd
pull-kubernetes-e2e-gce	c2c063d	link	true	/test pull-kubernetes-e2e-gce
pull-kubernetes-typecheck	c2c063d	link	true	/test pull-kubernetes-typecheck
pull-kubernetes-conformance-kind-ga-only-parallel	c2c063d	link	true	/test pull-kubernetes-conformance-kind-ga-only-parallel
pull-kubernetes-e2e-kind	c2c063d	link	true	/test pull-kubernetes-e2e-kind
pull-kubernetes-e2e-capz-windows-master	c2c063d	link	false	/test pull-kubernetes-e2e-capz-windows-master
pull-kubernetes-node-e2e-containerd	c2c063d	link	true	/test pull-kubernetes-node-e2e-containerd
pull-kubernetes-linter-hints	c2c063d	link	false	/test pull-kubernetes-linter-hints
pull-kubernetes-unit	c2c063d	link	true	/test pull-kubernetes-unit
pull-kubernetes-unit-windows-master	c2c063d	link	false	/test pull-kubernetes-unit-windows-master
pull-kubernetes-integration	c2c063d	link	true	/test pull-kubernetes-integration
pull-kubernetes-verify	c2c063d	link	true	/test pull-kubernetes-verify

Full PR test history. Your PR dashboard. Please help us cut down on flakes by linking to an open issue when you hit one in your PR.

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[aaron-prindle]

/cc @soltysh
/remove-sig api-machinery
/sig cli

[k8s-ci-robot]

PR needs rebase.

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes-sigs/prow repository.