Incorrect KDF salt type derived from principal name

**Describe the bug**
The KDF generates incorrect salt for principal *Administrator@Adatum.com*:

Actual result: *ADATUM.COMhostAdministrator.adatum.com*
Expected result: *ADATUM.COMAdministrator*

**To Reproduce**

```cs
var krbPrincipalName = PrincipalName.FromString(principal: "Administrator@Adatum.com", realm: "Adatum.com");
var principalName = PrincipalName.FromKrbPrincipalName(krbPrincipalName);
KerberosKey aes256sha1Key = new(password: "Password123", etype: EncryptionType.AES256_CTS_HMAC_SHA1_96, principal: principalName);
// Check aes256sha1Key.SaltFormat
// Check aes256sha1Key.Salt
```

(Also notice that `PrincipalName.FromString()` strangely returns `KrbPrincipalName` instead of `PrincipalName`, so an additional conversion is needed.)

**Expected behavior**

If `KerberosKey.SaltFormat` is not provided to the `KerberosKey` constructor by the caller, it should be derived from `PrincipalName.Type`. It is instead always `SaltType.ActiveDirectoryService`, even for `PrincipalNameType.NT_PRINCIPAL`, although it should be `SaltType.ActiveDirectoryUser` in this case.