tls_private_key resource: adding support for ED25519 key algorithm

[detro]

Context: #150

This PR starts from the work of #59 (thank you!) and moves towards the use of upcoming features of x/crypto/ssh (link).

A configuration like this will now be possible:

resource "tls_private_key" "test" {
    algorithm = "ED25519"
}

output "private_key_pem" {
    value = "${tls_private_key.test.private_key_pem}"
    sensitive = true
}

output "private_key_openssh" {                                      #< new
    value = "${tls_private_key.test.private_key_openssh}"
    sensitive = true
}

output "public_key_pem" {
    value = "${tls_private_key.test.public_key_pem}"
}

output "public_key_openssh" {
    value = "${tls_private_key.test.public_key_openssh}"
}

output "public_key_fingerprint_md5" {
    value = "${tls_private_key.test.public_key_fingerprint_md5}"
}

output "public_key_fingerprint_sha256" {.                           #< new
    value = "${tls_private_key.test.public_key_fingerprint_sha256}"
}

Subtasks in this PR:

• New private_key_openssh for all key algorithm that we can generate OpenSSH PEM format for (i.e. this excludes ECDSA P-224, because of SSH limitations)
• New public_key_fingerprint_sha256 for all key algorithms that we can generate OpenSSH PEM format for (same as above)
• Extend test-set to cover the new fields
• Update tls_private_key documentation
• Extend testing matrix to include all current minor versions of Terraform

PLEASE NOTE: This PR does NOT cover everything we want to address in #150 but focuses around tls_private_key. This means that we will not be cutting a release until all the items in #150 are addressed.

[kmoe]

Adding comments from verbal review. Excellent work and LGTM in general with minor changes.

[bflad]

Looking good so far 😄 Just a few things to consider while we are in the neighborhood.

[cipherboy]

Nothing major from my end, just a couple of nits.

[cipherboy]

Looks like all my changes are addressed.

[github-actions]

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