Mail Service Impl

[hhund]

Adds a service implementation to send e-mails from DSF processes:

• If not configured, generated e-mail will be logged
• SMTP via TLS (smtps) can be enabled
• Username/Password or client-certificate authentication to the SMTP server can be configured
• A certificate trust store for the SMTP server can be configured
• A S/MIME certificate can be configured to send signed e-mails
• The BPE will send a test mail on startup if enabled
• The BPE will send mail with Log-Message on ERROR including the last 4 INFO and WARN Messages if enabled
• SMTP via HTTP or SOCKS proxy is currently not implemented

closes #370