Extend parser to allow custom promise types via promise modules

Started here:

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

Acceptance criteria:

• Parser allows new top level promise block for adding new promise types (via modules)
• Parser accepts new promise types without erroring (after they have been added with promise block)
• Parser errors if you try using a promise type without adding it first.
• Agent starts a promise module process and communicates with it using promise module protocol
• Custom promises can be validated and evaluated, according to spec
• Attempting to use one of the common attributes which is not implemented yet should result in error

Specification:

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