Add configurable max message size enforcement

[tinohager]

Introduces IMaxMessageSizeOptions, MaxMessageSizeOptions, and MaxMessageSizeHandling to allow configuration of maximum allowed message size and handling strategy. Updates relevant protocol and IO classes to enforce the limit, throwing MaxMessageSizeExceededException when exceeded in strict mode. Adjusts SmtpServerOptionsBuilder and ISmtpServerOptions to use the new options, and adds a test for strict enforcement.

Based on this existing pull request of @boba2fett

• Add Resilience Features #213

Fix the issue

• MaxMessageSize not checked after DATA #225

The code should be largely backward compatible since the second parameter was declared as optional in the builder.

SmtpServerOptionsBuilder MaxMessageSize(int value)
SmtpServerOptionsBuilder MaxMessageSize(int length, MaxMessageSizeHandling handling = MaxMessageSizeHandling.Ignore)

An example of the server configuration (Allow max 50 bytes)

var options = new SmtpServerOptionsBuilder()
    .ServerName("SMTP Server")
    .Port(9025)
    .MaxMessageSize(50, MaxMessageSizeHandling.Strict)
    .Build();

[tinohager]

552 message size exceeds fixed maximium message size <https://datatracker.ietf.org/doc/html/rfc1653> [ietf-logo-card.png] RFC 1653: SMTP Service Extension for Message Size Declaration<https://datatracker.ietf.org/doc/html/rfc1653> datatracker.ietf.org<https://datatracker.ietf.org/doc/html/rfc1653>