Add SSH key pair generation

[MansiVisuals]

Adds MFTKeyGenerator, for generating an SSH key pair locally. No connection is
involved: this is key material made before there is anything to connect to, so
an app can offer "generate a key" without first having a server to talk to.

Ed25519, RSA 4096 and ECDSA P-256 are supported. The pair comes back as an
MFTKeyPair holding the PEM wrapped private key and a single authorized_keys
line for the public half, so the public half can be shown or copied directly.
Passing a passphrase encrypts the private half.

let pair = try MFTKeyGenerator.generate(type: .ed25519, passphrase: "secret")
print(pair.publicKey)   // ssh-ed25519 AAAAC3Nza... 
print(pair.keyType)     // ssh-ed25519

Two details worth flagging:

• The RSA modulus size is requested through an ssh_pki_ctx. Passing a null
  context to ssh_pki_generate_key leaves libssh at its 3072 bit default, so
  .rsa4096 would otherwise quietly produce a 3072 bit key.
• Ed25519 private keys come out in OpenSSH format, RSA and ECDSA ones in
  PKCS#8. That is libssh's behaviour; it is documented on the property rather
  than normalised.

Everything is @objc exposed, matching the rest of the framework.

Verified by generating all three types, writing them out, and checking them with
ssh-keygen: the reported sizes are 256 / 4096 / 256, each public line matches
the one ssh-keygen -y derives from the corresponding private key, and a
passphrase protected key fails to load with the wrong passphrase and succeeds
with the right one.

[mplpl]

By adding new class here, you separated the comment above (starting line 66) from the function (MFTSftpConnection) it refers to. This needs to be fixed.

Also, these new classes could be moved to a new file as then are not directly related to MFTSftpConnection implemented in this file.

[MansiVisuals]

Appreciate the fast review of both PR's

I will adjust this PR, so it's corrected and not writing them into the existing MFTSftpConnection when not needing too.

Thank you will get back to me you asap

[MansiVisuals]

Moved the new classes to their own file, mft/MFTKeyGenerator.swift. MFTSftpConnection.swift is back to how it was and does not show up in the diff at all now, so the comment is back where it should be.

Added the file to both the mft and mft ios targets, both build fine here.

[mplpl]

Thanks for contributing!