feat(appkit): plugin manifest definition

[MarioCadenas]

Summary

This PR introduces a plugin manifest definition and resource registry so plugins can declare their Databricks resource requirements in a machine-readable way. Tooling can then derive app configuration (e.g. app.yaml / bundle resources) from those manifests.

Plugin manifest

• Manifest is required: Every plugin must define a static manifest with name, displayName, description, and resources (required/optional).
• Manifest format: JSON Schema in plugin-manifest.schema.json; built-in plugins (analytics, server) ship a manifest.json next to their code and a typed manifest.ts used at runtime.
• Runtime requirements: Plugins can implement static getResourceRequirements(config) so optional resources can become required based on config (e.g. warehouse only when analytics is enabled).

Resource registry

• ResourceRegistry: Collects resource requirements from all plugins at app creation, deduplicates by type + resourceKey, and merges permissions (e.g. takes the most permissive when several plugins need the same resource).
• Resource types: Typed enum (ResourceType) and per-type permissions (e.g. sql_warehouse → CAN_USE / CAN_MANAGE) in packages/appkit/src/registry/types.ts.
• Validation: createApp() calls registry.collectResources(rawPlugins) then registry.enforceValidation() so missing or invalid manifests fail fast.

Manifest loader

• getPluginManifest(plugin): Loads and validates the manifest from a plugin constructor; normalizes string type/permission into ResourceType / ResourcePermission.
• getResourceRequirements(plugin, config?): Returns the flattened list of requirements (manifest + optional runtime from getResourceRequirements(config)).

CLI: appkit plugins sync

• New command in @databricks/shared that discovers plugins from the app's server entrypoint (AST parsing of imports and plugin usage), reads their manifests from installed packages or local paths, and builds/updates appkit.plugins.json.
• Template uses this file (and the new template-plugins.schema.json) so the template's plugin list and resource metadata stay in sync with the app.
• Options: --write to write the file, --output <path> for custom path.

Template and docs

• Template: appkit.plugins.json added; analytics bundle config simplified (fewer redundant YAML files); databricks.yml.tmpl and server wiring updated to use the new plugin manifest story.
• Docs: Plugin docs and API reference updated (Plugin class, PluginManifest, ResourceRegistry, ResourceType, new functions), plus new schema docs and copy script for static schemas.

Other

• Dev-playground: Reconnect and telemetry-example plugins updated to define a manifest.
• Removals: env-validator and its exports removed from appkit.
• Tests: New/updated tests for manifest loader, resource registry (including getResourceRequirements and permission merging), integration with createApp, and plugins-sync CLI.

[pkosiec]

Overall LGTM, just small comments

[pkosiec]

Approving as I already checked the PRs which were merged into this one