Support cards / complex data elements inside element templates

__Is your feature request related to a problem? Please describe__

Often times I'm dealing with complex data in my element template. Considering a template for a Slack connector for example: While some of the data (i.e. a users name and avatar) is important for me to understand the property I configured other data is required for execution (the user or channel ID).

Right now I can only input plain data, which is for example a users technical name (or `id`). It may not mean a lot to me (often times missing contextual help / avatar / human readable label):


__Describe the solution you'd like__

Rather than configuring plain text (the user ID) I'd love to pick (https://github.com/bpmn-io/element-templates/issues/1) and see the complex data element that I've configured:


That data shall be persisted during diagram safe (i.e. I want to see it again next time I configure the task.

The basic requirements for the complex data element are:

* Visual `label`
* Technical meta-data `value` ?
* Optional `icon`

Potential candidates:

* Optional long form `description`
* Optional `url` to link to

__Describe alternatives you've considered__

Keep templates static and _plain_.

__Additional context__

Related to https://github.com/bpmn-io/element-templates/issues/1 (we shall support the same complex data there).

Start by reviewing related issue #1 and the examples in this request. Define how complex elements expose label, technical value, and optional icon or metadata, and how the selected data is persisted and shown again when configuring the task. Done means templates support the requested complex data without losing its execution value.

Written by the indexing model from the issue text.