Enhance environment variable retrieval and defaults

- Modify `Helper.cs` to retrieve `EmailAccountPassword` from process-level environment variables and set a default SMTP port of `25`.
- Update `EnvHelper.cs` `Get` method to support a `defaultValue` parameter, returning it if the environment variable is not found.

src/Moonglade.Function.Email/Core/Helper.cs

@@ -12,8 +12,8 @@ public static EmailHelper GetEmailHelper(string functionAppDirectory)
 
         var smtpSettings = new SmtpSettings(EnvHelper.Get<string>("SmtpServer"),
             EnvHelper.Get<string>("SmtpUserName"),
-            EnvHelper.Get<string>("EmailAccountPassword", EnvironmentVariableTarget.Process),
-            EnvHelper.Get<int>("SmtpServerPort"));
+            EnvHelper.Get<string>("EmailAccountPassword", target: EnvironmentVariableTarget.Process),
+            EnvHelper.Get("SmtpServerPort", 25));
 
         if (EnvHelper.Get<bool>("EnableTls"))
         {

src/Moonglade.Function.Email/EnvHelper.cs

@@ -7,7 +7,7 @@ public static class EnvHelper
 {
     public static ICollection AllKeys => Environment.GetEnvironmentVariables().Keys;
 
-    public static T Get<T>(string name, EnvironmentVariableTarget? target = null)
+    public static T Get<T>(string name, T defaultValue = default, EnvironmentVariableTarget? target = null)
     {
         if (string.IsNullOrEmpty(name))
         {
@@ -17,7 +17,7 @@ public static T Get<T>(string name, EnvironmentVariableTarget? target = null)
         string matchedKey = AllKeys.Cast<string>().FirstOrDefault(key => string.Equals(key, name, StringComparison.OrdinalIgnoreCase));
         if (matchedKey == null)
         {
-            return default;
+            return defaultValue;
         }
 
         var value = target == null