Add support for the fields property of attachments as documented here:

https://rocket.chat/docs/developer-guides/rest-api/chat/postmessage/#attachment-field-objects

Possible implementation:
Create a new class named Field here: https://github.com/jenkinsci/rocketchatnotifier-plugin/tree/develop/src/main/java/jenkins/plugins/rocketchatnotifier/model

The java property (field + getter/setters) of type List<Field> with a proper @JsonProperty annotation would need to be added to: https://github.com/jenkinsci/rocketchatnotifier-plugin/blob/develop/src/main/java/jenkins/plugins/rocketchatnotifier/model/MessageAttachment.java

The rest of the code will likely remain untouched since it's a property of MessageAttachment which is already used implicitly in classes like RocketChatClientImpl and RocketChatClientCallBuilder