Schema Validation v2

_(this is the next feature increment of #561)_

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**As a** Configuration Author
**I want to** declare what constitutes a _valid_ Data Value right in my schema definition
**In a way that** requires even less custom Starlark code from me ⭐
**And** I can write fewer annotations ⭐ because `ytt` can confidently infer for what _have_ written.
*So that**
- I can state those invariants in one central place and know they will be respected throughout the execution
- those who use my `ytt` library and violate one of those variants will receive a helpful error message
- and my overall maintenance effort/costs are less than before. ⭐

_(⭐ = a desired improvement on top of the MVP.)_

[[proposal]](https://github.com/vmware-tanzu/carvel/pull/331/files?short_path=a81faf6#diff-a81faf6d33e82e22eee1083ee484a23eda8a928b046116a2cb516e58086984b8)
[[getting started]](https://carvel.dev/blog/ytt-validations-preview/)

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### Backlog
- _⚠️ This track of work is still being elaborated; some scope is likely missing.⚠️_
- loads of ideas for improvements can be mined from #707's [synthesis](https://miro.com/app/board/uXjVOgHIamo=/)

UX:
- [ ] Use custom violation messages
- on `@assert/validate`, implement 3-tuple rules where the third tuple is the violation template; to `@schema/validation`, add 3-tuple tules.
- note: this is critical to improve the DX around unit testing (detailed in [a conversation on #741](https://github.com/vmware-tanzu/carvel-ytt/pull/741#discussion_r969654801))
- [ ] disallow `@schema/validation` in non-schema documents.
- [ ] disallow `@assert/validate` in schema documents.

Fast Follow: other must-have OOTB rules
- [ ] `matches=`

Programmatic Validation of YAML
- [ ] Validate a YAML document that contains `@assert/validate` annotations
- expose `assert.is_valid()` as a method in `@ytt:assert` module

Include Validations in Schema Inspect
- [ ] Include validations when exporting schema to OpenAPI v3 (subset: one-for-ones)
- need: https://kubernetes.slack.com/archives/CH8KCCKA5/p1661186311752459?thread_ts=1661185491.294779&cid=CH8KCCKA5
- [ ] Include validations when exporting schema to OpenAPI v3 (subset: needing translation)

Authoring Conveniences:
- [ ] Set a default validation for descendent Data Values (`@schema/validate-string-default`)
- [ ] Infer `@schema/nullable` for all children implicated in a `@schema/validation one_not_null=`

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### Resources
- [Schema proposal: Part 7 — Validating Documents](https://hackmd.io/pODV3wzbT56MbQTxbQOOKQ#Part-7-Validating-Documents)
- [Schema Validation proposal](https://github.com/vmware-tanzu/carvel/pull/331/files?short_path=a81faf6#diff-a81faf6d33e82e22eee1083ee484a23eda8a928b046116a2cb516e58086984b8)
- Previous Epic: #561

### Related Issues:
- #103 — originating issue
- #556 — requesting "required" use-case, explicitly.

Start with the Schema Validation proposal, the getting-started article, and the linked synthesis from #707. Review the backlog and related issues #103 and #556 to determine which increment has agreed scope. Done criteria are not defined here because the track is still being elaborated.

Written by the indexing model from the issue text.