Normalize provided private key string to the correct PEM format

[ykitamura-mdsol]

Storing a private key string in an environment variable could be challenging with the correct PEM format (having newline characters).

With this PR, the provided private key string will be normalized to the correct PEM format.

[gitguardian]

⚠️ GitGuardian has uncovered 1 secret following the scan of your pull request.

Please consider investigating the findings and remediating the incidents. Failure to do so may lead to compromising the associated services or software components.

🔎 Detected hardcoded secret in your pull request

GitGuardian id	GitGuardian status	Secret	Commit	Filename
14950534	Triggered	RSA Private Key	2506152	src/pem_format.rs	View secret

🛠 Guidelines to remediate hardcoded secrets

1. Revoke and rotate the secret.

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^{🦉 GitGuardian detects secrets in your source code to help developers and security teams secure the modern development process. You are seeing this because you or someone else with access to this repository has authorized GitGuardian to scan your pull request.}