Plugin type to allow multiple elements to be declared in a single .bst file

[See original issue on GitLab](https://gitlab.com/BuildStream/buildstream/-/issues/1327)
In GitLab by [[Gitlab user @tristanvb]](https://gitlab.com/tristanvb) on May 28, 2020, 11:40

In order to support reusability of more advanced constructs, we need a way to support express multiple elements in a single `.bst` file, so far the best idea for this is to introduce a new `Plugin` subclass allowing third parties to implement a `Composite` plugin type (or a differently named thing), which has it's own YAML configuration surface and allows expressing multiple elements which might have implied dependency relationships with one another.

This was discussed [a long time ago](https://mail.gnome.org/archives/buildstream-list/2018-October/msg00077.html) in the context of *"testing elements"*, and elaborated in a Manchester BeaverCon, and has recently been [brought up again](https://mail.gnome.org/archives/buildstream-list/2020-May/msg00016.html).

Note that this is on the `2.0` milestone list for now but can be removed from the milestone:

* If a well thought out proposal is brought to the mailing list and linked to this issue
* If it is agreed that an implementation of the proposal is both suitable, and can be done without breaking API.

Start with the linked GitLab issue and the referenced mailing-list discussions from October 2018 and May 2020. Develop a concrete proposal for a Plugin subclass that can express multiple elements and their implied dependencies, then seek agreement that the design is suitable and does not break the API; an accepted proposal or implementation would define done.

Written by the indexing model from the issue text.