PostalAPI Service missing expansions argument on details function

Problem/Motivation

We use postal integration in our project and for several separate tasks we use the PostalAPI service
to extract detailed information about a given email.
The problem arises when I try to get the information through the details($id) function from the service, but
in the response, some fields such as "plain_body" have a NULL value.
According to the official Postal documentation > Postal Docs, one can send so-called "expansions" to define which blocks should be present in the response.
With this patch, I am simply adding the missing argument so that a programmer can correctly extract everything they need.

Steps to reproduce

1. Create a custom event subscriber and subscribe to the Webhook
2. Try to get details about the incoming/outgoing message through PostalAPI service

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes