feature: add support to specify IDs for entities

[hbagdi]

Currently, IDs cannot be specified in decK for any entitiy, and if they are specified, they are just ignored.
This feature will also benefit Kong's Ingress Controller as the IDs can be deterministic.

[hbagdi]

Although, this sounds simple enough, it is much more complicated.

The redesign impacts all areas and will make the codebase uniform, opening up the doors to auto-generating most parts of decK, simplifying maintenance and adding support for further entities and even making decK work for other RESTful APIs.

Here are the broken down tasks (these might not be granular enough):

• Use ID based foreign references.
• Update targetstate to match IDs with the current state and perform a diff including the ID. Autogen a UUID when one doesn't exist.
• The schema is to be generated automatically using the Content struct. Use CI to detect any drift. This also opens up door for feature request: add support for reading JSON files #35
• JSONschema based input file validation.