feat: implement missing data source github_actions_environment_public_key

[nobbs]

Resolves #1769 (it was closed but never resolved)

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Before the change?

• there are multiple data source implementations to get the public key used for secrets encryption, e.g.
  • github_actions_public_key for a repository's public key
  • github_actions_organization_public_key for an organization's public key
• there is no data source to get the public key of a specific environment of a repository (which might be used to encrypt values for the github_repository_environment_secret resource).

After the change?

• implements the missing data source github_actions_environment_public_key

data "github_actions_environment_public_key" "test" {
  repository = "repo-name"
  environment = "env-name"
}

Pull request checklist

• Tests for the changes have been added (for bug fixes / features)
• Docs have been reviewed and added / updated if needed (for bug fixes / features)

Does this introduce a breaking change?

Please see our docs on breaking changes to help!

• Yes
• No

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