russh doesn't parse comments from public key files

[KazWolfe]

Description

When parsing a public key file with load_public_key(path), the key's comment is not returned in the final PublicKey struct.

Steps To Reproduce

1. Create a keypair with a comment, e.g. ssh-keygen -t ed25519 -C "my comment" -f keypair
2. Dump key information with appropriate functions (see sample code).
3. Observe error.

Sample Code

fn main() {
    let key = load_public_key("./keypair.pub").unwrap();
    println!("Key Type: {}", key.algorithm().to_string());
    println!("Key Bytes: {}", key.to_string());
    println!("Key Comment: {} (empty: {})", key.comment().to_string(), key.comment().is_empty());
    println!("\nEncoded Key Format:\n\t{}", key.to_openssh().unwrap());
}

Sample Execution

➜  debug git:(master) ✗ ssh-keygen -t ed25519 -C "my comment" -f keypair
Generating public/private ed25519 key pair.
[ snip ]
➜  debug git:(master) ✗ ./russh-demo                                    
Key Type: ssh-ed25519
Key Bytes: ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAILgm39eVWrPRMq8Kl07z6GFZyfXD4FdVcelOEdH0PVGc
Key Comment:  (empty: true)

Encoded Key Format:
        ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAILgm39eVWrPRMq8Kl07z6GFZyfXD4FdVcelOEdH0PVGc

➜  debug git:(master) ✗ cat keypair.pub 
ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAILgm39eVWrPRMq8Kl07z6GFZyfXD4FdVcelOEdH0PVGc my comment

Expected Results

The comment should be present in both key.comment() as well as in the re-encoded .to_openssh() method.

Actual Results

The comment appears to never be parsed.

Other Notes

A similar phenomenon can also be seen with AgentClient<?>.request_identities(). In that case, however, the returned AgentIdentity will populate the comment, but the underlying public_key is still missing it.