rust: enhance email functionality

[charmitro]

This update significantly enhances the interface for sending emails from C to Rust by introducing a structured approach with the EmailInfo struct. The EmailInfo struct formalizes the parameters for sending an email, including sender, recipients, CC, and body, making it easier for C developers to utilize the Rust email sending functionality.

Key changes include:

• Introduction of the EmailInfo struct in C bindings, providing a clear and structured way to pass email information from C to Rust.
• Detailed documentation comments added to email-bindings.h to guide C developers on using the EmailInfo struct and the send_email function, with example usage that demonstrates constructing the EmailInfo struct and invoking the email sending process.
• Refinement of the Rust send_email function to accept an EmailInfo struct as its parameter, streamlining the process of email construction and sending within the Rust code.

[charmitro]

Converting to draft, I'm missing SMTP server options.