SS-8 Add DSN envelope parameter support

[vhafdal]

Summary

• Parse and preserve DSN-related MAIL parameters such as RET and ENVID.
• Parse and preserve RCPT parameters such as NOTIFY and ORCPT.
• Expose accepted recipients and their RCPT parameters through optional transaction interfaces and GetRecipients().
• Add an optional parameter-aware mailbox filter interface for RCPT-time filtering.
• Advertise DSN in EHLO and document the implementation limitation that applications remain responsible for generating delivery status notifications.
• Add parser and raw SMTP integration coverage for DSN envelope parameters.

Verification

• DOTNET_ROLL_FORWARD=Major dotnet test src/SmtpServer.Tests/SmtpServer.Tests.csproj --no-restore
  • Passed: 120, Skipped: 1
  • Existing warnings observed: MailKit NU1902 advisory, existing AuthCommand XML doc warning on upstream master, existing xUnit analyzer warnings.

[vhafdal]

Superseded by consolidated PR #284.