Annotate Data Value Schema with validation

[pivotaljohn]

Given a Data Values schema, annotated with a validation rule:

#! schema.yml

#@ load("@ytt:assert", "assert")
@data/values-schema
---
#@schema/validation ("a non-empty string", lambda v: assert.fail("length = {}", len(v)) if len(v) > 0 else None)
username: ""

Details at Proposal > Specification > @schema/validation.

Scenario: Invalid Data Value

Given the final Data Value's value does not satisfies the rule:
When ytt runs
Then reports a Data Values violation error message:

$ ytt -f schema.yml --data-values-inspect
ytt: Error: One or more data values were invalid:
- "adminPort" requires a valid value: a non-empty string; length = 0 (by schema.yml:6).

Details at Proposal > Specification > @schema/validation.

Scenario: Valid Data Value

Given the final Data Value's value satisfies the rule:
Then no error message is displayed.

$ ytt -f schema.yml -v username=alice --data-values-inspect
username: alice

---

Notes:

• ideally, this work builds off of Annotate a Data Value with validation #604.