Add custom attributes for brevo

[ammannbe]

First, thank you for this plugin!

I opened this PR to allow custom attributes for the brevo integration.
The field can dynamically provided by the frontend, or left away completely.

[juban]

Hi @ammannbe
Thank you for your PR which looks good.
I'll take a look as soon as possible.

[juban]

Hi @ammannbe

Sorry for the delay.
Based on your PR, I would like to give the feature a more "provider agnostic" approach.
I'm in the process of adding the "additional fields" capability to both Mailchimp and Mailjet adapters.
I'll also have to adapt the tests in order to take the new feature into account and write according documentation.

Thanks for your patience.