add sshkey package and ssh-keygen comand

[azr]

This is preliminary work for #10074 next: refactor plugins that generate their own ssh keypair to start using this.

This brings:

• a helper/communicator/sshkey package that has a sshkey.GeneratePair function that will allow to generate valid open ssh keypairs.
• a cmd/ssh-keygen package that is a simplified go version of ssh-keygen to test the pairs with real ssh servers.
• go tests to see wether the keypairs can be parsed, although a 'real' ssh server/client is a much better way to test.

[codecov]

Codecov Report

Merging #10101 into master will increase coverage by 0.10%.
The diff coverage is 68.50%.

Impacted Files	Coverage Δ
helper/communicator/sshkey/algorithm_enumer.go	11.11% <11.11%> (ø)
helper/communicator/sshkey/generate.go	74.17% <74.17%> (ø)
packer/communicator.go	76.59% <0.00%> (+2.12%)	⬆️

[SwampDragons]

Cool! This is a bit lower-level than I'd expected you'd have to get. But looks good. Can't wait to see it in action.

[azr]

Same here ! Hopefully the cryptography is still handled by the "crypto" packages and I just had to work on wrapping/unwrapping 🙂 .

[ghost]

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