Custom promise type design, specification and prototype

Specification:

https://github.com/cfengine/core/blob/master/docs/custom_promise_types/modules.md

Old proposal:

https://github.com/tzz/core/blob/tzz/RFC-cf-module/ext/cf-module.md

Considerations:

• It is useful to "scope down", i.e. limit this to only promise types. (While we would like to add functions and other things later).
• The proposal above uses non-standard JSON (keys without quotes), we should stick to the standard.

Acceptance criteria:

• Basic design document which explains the design and protocol, and how to add a promise module.
• Possibility for users to add new promise types without rebuilding, for example in a Python script.
• Initial prototype PR which implements modules which can be tested and reviewed / iterated on.
• Example promise module written in python

PR with prototype:

https://github.com/cfengine/core/pull/4137