feat: Add key-value settings to plugin input

[adamspofford-dfinity]

Looks like:

type: plugin
fields:
  a: b

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[Copilot]

Pull request overview

Adds manifest-defined key-value fields to sync plugin inputs.

Changes:

• Parses scalar fields values and passes them through the WIT runtime.
• Preserves fields during project bundling.
• Adds tests, schemas, and documentation.

Reviewed changes

Copilot reviewed 14 out of 14 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
docs/schemas/icp-yaml-schema.json	Adds fields schema.
docs/schemas/canister-yaml-schema.json	Adds fields schema.
docs/reference/configuration.md	Documents configuration.
docs/guides/writing-sync-plugins.md	Documents plugin usage.
docs/concepts/sync-plugins.md	Describes runtime behavior.
crates/icp/src/manifest/canister.rs	Updates test expectations.
crates/icp/src/manifest/adapter/plugin.rs	Parses and validates fields.
crates/icp/src/canister/sync/plugin.rs	Forwards fields to runtime.
crates/icp-sync-plugin/tests/fixtures/test-plugin/src/lib.rs	Exercises field input.
crates/icp-sync-plugin/sync-plugin.wit	Extends plugin input contract.
crates/icp-sync-plugin/src/runtime.rs	Passes fields into plugins.
crates/icp-sync-plugin/DESIGN.md	Updates design documentation.
crates/icp-cli/tests/bundle_tests.rs	Tests bundle preservation.
crates/icp-cli/src/operations/bundle.rs	Preserves fields in bundles.

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