Add declarative secrets schema

[joshspicer]

Updates schema to eliminate the invalid/confusing linter errors with declarative secrets.

This file is referenced by VS Code.

Related: https://docs.github.com/en/codespaces/setting-up-your-project-for-codespaces/configuring-dev-containers/specifying-recommended-secrets-for-a-repository#specifying-recommended-secrets-in-the-dev-container-configuration

Demo

https://github.com/devcontainers/spec/assets/23246594/dcf7b282-57d3-421e-bc21-5fc22e92d193

[haticecan37]

{ "allOf": [ { "$ref": "./devContainer.base.schema.json" }, { "$ref": "https://raw.githubusercontent.com/microsoft/vscode/main/extensions/configuration-editing/schemas/devContainer.codespaces.schema.json" }, { "$ref": "https://raw.githubusercontent.com/microsoft/vscode/main/extensions/configuration-editing/schemas/devContainer.vscode.schema.json" }

debug.log ] }