support a config method to set the KeychainConfiguration

Sources/Vault/Vault.swift

···
     var accountName: String { get set }
 }
 
+public enum VaultError: Error {
+    case noKeychainConfiguration
+}
+
+extension VaultError: LocalizedError {
+    public var errorDescription: String? {
+        switch self {
+        case .noKeychainConfiguration:
+            "Vault error: A KeychainConfiguration object is needed to complete action. Please provide one by either passing it to this method or by using the Vault configure method"
+        }
+    }
+}
+
 public struct Vault {
+    static var keychainConfiguration: KeychainConfiguration?
     
-    public static func savePrivateKey(_ privateKey: String, keychainConfiguration: KeychainConfiguration) throws {
+    public static func configure(_ keychainConfiguration: KeychainConfiguration) {
+        Vault.keychainConfiguration = keychainConfiguration
+    }
+    
+    private static func getKeychainConfiguration(passedInParameterValue value: KeychainConfiguration?) throws -> KeychainConfiguration {
+        if let value {
+            return value
+        } else if let keychainConfiguration {
+            return keychainConfiguration
+        } else {
+            throw VaultError.noKeychainConfiguration
+        }
+    }
+    
+    public static func savePrivateKey(_ privateKey: String, keychainConfiguration: KeychainConfiguration? = nil) throws {
+        let keychainConfiguration = try getKeychainConfiguration(passedInParameterValue: keychainConfiguration)
         try KeychainItem(service: keychainConfiguration.serviceName, account: keychainConfiguration.accountName, accessGroup: keychainConfiguration.accessGroup).saveItem(privateKey)
     }
     
-    public static func getPrivateKey(keychainConfiguration: KeychainConfiguration) throws -> String {
+    public static func getPrivateKey(keychainConfiguration: KeychainConfiguration? = nil) throws -> String {
+        let keychainConfiguration = try getKeychainConfiguration(passedInParameterValue: keychainConfiguration)
         return try KeychainItem(service: keychainConfiguration.serviceName, account: keychainConfiguration.accountName, accessGroup: keychainConfiguration.accessGroup).readItem()
     }
     
-    public static func deletePrivateKey(keychainConfiguration: KeychainConfiguration) throws {
+    public static func deletePrivateKey(keychainConfiguration: KeychainConfiguration? = nil) throws {
+        let keychainConfiguration = try getKeychainConfiguration(passedInParameterValue: keychainConfiguration)
         try KeychainItem(service: keychainConfiguration.serviceName, account: keychainConfiguration.accountName, accessGroup: keychainConfiguration.accessGroup).deleteItem()
     }
 }