Shared Tree: Simple Schema codec

[TommyBrosman]

Introduces new APIs for encoding and decoding view schemas. Also adds new fields to Simple Schemas to support compatibility checking scenarios.

Description

New APIs (in alpha):

• encodeSimpleSchema(simpleSchema: SimpleTreeSchema): JsonCompatibleReadOnly - Encodes a Simple Schema into a JSON-compatible object format.
• decodeSimpleSchema(encodedSchema: JsonCompatibleReadOnly, validator?: FormatValidator): SimpleTreeSchema - Decodes a Simple Schema from a JSON-compatible object format.

Other changes:

• Updated toSimpleTreeSchema to include `compatibility-related fields. These include:
  • On SimpleAllowedTypeAttributes: isStaged
  • On object schemas: allowUnknownOptionalTypes

Reviewer Guidance

• See discrepancies.ts for the set of fields needed to determine View Schema compatibility.

Todo

• Deserialization
• Fix test failures
• Expose APIs

[Josmithr]

Left a couple more suggestions, but overall looks good to me :)

[github-actions]

🔗 Found some broken links! 💔

Run a link check locally to find them. See
https://github.com/microsoft/FluidFramework/wiki/Checking-for-broken-links-in-the-documentation for more information.

linkcheck output

> fluid-framework-docs-site@0.0.0 ci:check-links /home/runner/work/FluidFramework/FluidFramework/docs
> start-server-and-test "npm run serve -- --no-open" 3000 check-links

1: starting server using command "npm run serve -- --no-open"
and when url "[ 'http://127.0.0.1:3000' ]" is responding with HTTP status code 200
running tests using command "npm run check-links"


> fluid-framework-docs-site@0.0.0 serve
> docusaurus serve --no-open

[SUCCESS] Serving "build" directory at: http://localhost:3000/

> fluid-framework-docs-site@0.0.0 check-links
> linkcheck http://localhost:3000 --skip-file skipped-urls.txt

 ELIFECYCLE  Command failed with exit code 1.