Add the ability to customize email notification signature

[yruslan]

Background

The current signature looks like:

It could be nice to customize it, to include the version of custom components, the team name, contact email address etc.

Feature

Add the ability to customize email notification signature.

Proposed Solution

Making it part of config might be restrictive if you want to include some runtime information such as Pramen version and custom components version. So the proposal here is to extend the notification builder:

trait NotificationBuilder {
  def addEntries(...): Unit

  def addDataFrameTable(...): Unit

  /** Sets the notification signature at runtime. Can be used from the custom startup/shutdown hook. */
  def setSignature(text: Seq[TextElement]): Unit
}