Fix code scanning alert no. 507: Use of insecure HostKeyCallback implementation

[seokho-son]

Fixes https://github.com/cloud-barista/cb-tumblebug/security/code-scanning/507

To fix the problem, we need to replace the insecure HostKeyCallback implementation with a secure one. The best way to do this is to use the ssh.FixedHostKey function, which validates the host key against a predefined allow list. This involves reading the allowed host key from a file and using it to create the ssh.ClientConfig.

1. Read the allowed host key from a file.
2. Parse the host key using ssh.ParsePublicKey.
3. Use the parsed key with ssh.FixedHostKey to set the HostKeyCallback field in ssh.ClientConfig.

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