Add `$schema` to pact files

[mefellows]

Adding the $schema keyword to JSON files provides additional support for the comprehension of pact files. For example, most IDEs will provide auto-completion and documentation. Adding a schema will help tool builders who wish to parse, manipulate or otherwise handle pact files in an unobtrusive way.

For each pact specification, we publish JSON Schemas to describe their shape (they could do with some additional human-readable descriptions).

Example:

We may also wish to add $id, but I don't believe this is strictly necessary to get the benefit we're after.

[rholshausen]

Ok, I'm out, I quite. I'll leave it for you lot to mess up.

Seriously, the Pact file format is meant to be an intermediate format for machines, not for people to edit.

[mefellows]

Adding a schema will help tool builders who wish to parse, manipulate or otherwise handle pact files in an unobtrusive way.

I'm not suggesting people edit it by hand. Having a $schema in the pact file itself will make it more obvious to tool builders that such a schema exists and that they can then use it to ensure they do it safely. The IDE support would be helpful in those situations to guide the developer. It also provides an avenue for other machine-only use cases, such as validation.