[Internal]: Scheduled reports new email fields and Mustache support

[umbopepato]

Description

What: we're adding 4 more fields to the scheduled reports flyout email notification section (cc, bcc, subject and message)
When: merging soon for serverless, releasing in 9.3 for stateful

Resources

Implementation PR elastic/kibana#242922
Tracking issue: https://github.com/elastic/kibana-team/issues/2084

Which documentation set does this change impact?

Elastic On-Prem and Cloud (all)

Feature differences

No difference

What release is this request related to?

N/A

Serverless release

First serverless release of December

Collaboration model

The documentation team

Point of contact.

Main contact: @umbopepato