Support pluggable app metadata providers

[HituziANDO]

Problem

Update checking is fixed to the iTunes Search API. Enterprise, internal, regional, or self-hosted update feeds cannot reuse the framework's version policy and presentation behavior.

Scope

• Promote AppMetadataProvider to a documented extension point.
• Define a small metadata value containing version, release notes, and optional store URL.
• Keep the App Store implementation as the default provider.
• Allow applications to inject a custom asynchronous provider.
• Preserve typed error and cancellation behavior.

Dependencies

• Extract UpdateCheckService and add a structured async API #23

Acceptance criteria

• A custom provider can drive update checking without modifying framework sources.
• The update service has no dependency on App Store response DTOs.
• Default App Store behavior remains unchanged.
• Provider failures are surfaced through the structured result API.

Tests

• Add a fake provider, default-provider integration fixtures, cancellation coverage, and malformed metadata coverage.

Relevant files

• Framework/Sources/ITunesSearchAPI.swift
• Framework/Sources/SUK.swift
• Framework/Sources/SwiftyUpdateKitConfig.swift
• README.md