Add Audience and Contact endpoints functionality.

[mariobot]

Hi @filipetoscano In this addition PR work on the functionalities to add Audience and Contacts to the endpoints provided by resend.

The following additions were made:

Audience

• Audience Models.
• Audience to ResendClient.
• AudienceCommand.
• Audience Test

Contact

• Contact Models
• Contact to ResendClient
• ContactCommand
• Contact Terst

I am open to receiving any comments or recommendations in this regard and also if everything is correct see my changes in the master branch.

[filipetoscano]

@mariobot Thanks for the PR. My apologies for the delay, was busy at work.