underlyingCoreCryptoError while creating NIOSSHPrivateKey

Hi,
I'm trying to setup a SSH tunnel in order to forward a remote port into my local device. I followed your PR #55 and I successfully implemented it into a brand new iOS project: I am able to forward a port using password authentication.

What I'm trying to do is performing a private key authentication. Due to the lack of the documentation, I'm going to ask you a few questions. Here's the code:

``` swift
//ssh-keygen -t ecdsa -b 521 -m pem

let sshPrivateText = """
-----BEGIN EC PRIVATE KEY-----
row1
row2
row3
row4
row5
-----END EC PRIVATE KEY-----
"""

let base64EncodedString:String = Data(sshPrivateText.utf8).base64EncodedString()
let ecdsaPrivateKeyData:Data = Data(base64Encoded: base64EncodedString, options: .ignoreUnknownCharacters)!
let group = MultiThreadedEventLoopGroup(numberOfThreads: 1)
let key: NIOSSHPrivateKey?
do {
key = NIOSSHPrivateKey(p521Key: try .init(rawRepresentation: ecdsaPrivateKeyData))
} catch let error {
fatalError(error.localizedDescription)
}
```

But I can't go further the `NIOSSHPrivateKey` constructor since the .init throws an error:
```
Fatal error: The operation couldn’t be completed. (CryptoKit.CryptoKitError error 0.): file /Users/user/XCode Projects/NioExample/NioExample/ContentView.swift, line 35
2020-12-13 12:57:48.397737+0100 NioExample[2711:69134] Fatal error: The operation couldn’t be completed. (CryptoKit.CryptoKitError error 0.): file /Users/user/XCode Projects/NioExample/NioExample/ContentView.swift, line 35
(lldb)
```

```
▿ CryptoKitError
▿ underlyingCoreCryptoError : 1 element
- error : -1
```

Also, I have another question. Why the minimum SDK level is iOS13? Could it be downgraded or something? It's a huge device cut! Will this project be included in cocoapods? I'm asking this because I'm going to integrate this script into a native Flutter plugin and it seems like Flutter projects are not compatibile with SwiftPM.

Thank you for this amazing project by the way!

Start at the NIOSSHPrivateKey(p521Key:) call and compare the supplied PEM/Data conversion with the implementation referenced by PR #55. Reproduce the CryptoKit error, then establish the supported key representation and separately verify the iOS deployment target and package-distribution questions; the issue names no tests or source files.

Written by the indexing model from the issue text.