Add Field Group for Marketo Engage B2B data

## What are the schemas that are affected by the issue
This field group is meant to be used in the context of an [XDM Profile schema](https://github.com/adobe/xdm/blob/master/docs/reference/classes/profile.schema.md).

## What are examples of products that are impacted by the issue
Marketo Measure Ultimate and Marketo Engage integration will use this new field group to bring Marketo Engage data into Marketo Measure.

Start with the linked docs/reference/classes/profile.schema.md to understand the XDM Profile schema context. Then identify the required Marketo Engage data fields and how a field group is represented in this repository; the issue does not specify those fields or a target file. Done means a new field group is defined for Marketo Engage data and can be used with an XDM Profile schema.

Written by the indexing model from the issue text.