Jsonschema function for jsonnet

[ramaro]

Introduces new jsonschema function in jsonnet.

It can be used to validate the inventory with jsonschema.

Given this inventory:

mysql:                   
  storage: 10G           
  storage_class: standard
  image: mysql:latest    

The yaml inventory structure can be validated with the new jsonschema() function:

local schema = { type: "object",
                 properties: {
                   storage: { type: "string", pattern: "^[0-9]+[MGT]{1}$"},
                   image: { type: "string" },
                 }};
// run jsonschema validation
local validation = kap.jsonschema(inv.parameters.mysql, schema);
// assert valid, otherwise error with validation.reason
assert validation.valid: validation.reason;

If validation.valid is not true, it will then fail compilation and display validation.reason.

For example, if defining the storage value with an invalid pattern (10Z), compile fails:

Jsonnet error: failed to compile /code/components/mysql/main.jsonnet:
 RUNTIME ERROR: '10Z' does not match '^[0-9]+[MGT]{1}$'

Failed validating 'pattern' in schema['properties']['storage']:
    {'pattern': '^[0-9]+[MGT]{1}$', 'type': 'string'}

On instance['storage']:
    '10Z'

/code/components/mysql/main.jsonnet:(19:1)-(43:2)

Compile error: failed to compile target: minikube-mysql

Partially fixes issue #346

[uberspot]

Note: Add the example schema validation code in this PR to docs/compile.md for reference. :)
Other than that, lgtm!

[prakashrj]

Will it support reuse of object definitions like explained here.
https://json-schema.org/understanding-json-schema/structuring.html

We have been using RAML for validations. If reuse is supported, then we will use this feature to validate.

[ramaro]

@prakashrj not that the moment, but we can look into extending support.