Overhauled the library with several new features (#5)

## Summary

Updates the library to v4.5.1 of the App Store Connect API, and ships it as a breaking 2.0.0 release with pagination, API key authentication, rate limiting, and a lighter install for consumers.

## Breaking changes

- **Minimum platforms:** iOS 14, macOS 11, tvOS 14, watchOS 7, and visionOS 1.
- **Toolchain:** Swift 6.0+ (Xcode 16+). The package builds in the Swift 6 language mode.
- **Runtime:** requires `swift-openapi-runtime` 1.11.0 or later.

## Changes

- **API:** updated the OpenAPI specification to v4.5.1, covering 1,303 operations and 1,473 schemas.
- **Pagination:** the spec patch adds the undocumented `cursor` query parameter to the 465 paged operations. `PagedDocumentLinks.nextCursor` reads the cursor from a page, and `PagedSequence` iterates every page.
- **Authentication:** `APIKeyTokenProvider` signs ES256 tokens from an API key and reuses each one until a minute before it expires. `BearerAuthMiddleware` now accepts a token provider.
- **Rate limiting:** `RateLimitMiddleware` retries `429` responses, waiting for `Retry-After` or backing off exponentially.
- **Generated code:** committed to `Sources/ASConnectService/Sources/Public/Generated` instead of generated by a build plugin. Consumers now resolve 4 packages instead of 12, and Xcode no longer asks to trust a plugin.
- **Tooling:**
  - `make oas-download` now downloads, patches, generates, and checks the docs in one go.
  - New `make oas-generate`, `make oas-validate`, and `make doc-check` targets.
  - The generator is pinned in `Tools/`, and the DocC plugin is only resolved by the documentation targets.
- **Documentation:** the DocC catalog now lists every operation and schema in the spec, and the catalog and README are rewritten for the new APIs.
- **Housekeeping:** Apache 2.0 license headers on every Swift file, including `Package.swift` and the tests.

Reviewed-on: #5
Co-authored-by: Javier Cicchelli <javier@rock-n-code.com>

Sources/ASConnectService/Catalogs/ASConnectService.docc/Extensions/Servers.md

@@ -1,18 +1,15 @@
 # ``Servers``
 
-Server URLs defined in the OpenAPI document.
+The App Store Connect API server URLs.
 
 ## Overview
 
-The ``Servers`` namespace provides access to the base URLs defined in the App Store Connect API OpenAPI specification. Use these URLs when creating a ``Client`` instance.
+The specification defines a single server, `https://api.appstoreconnect.apple.com/`. Pass its URL to ``Client``:
 
 ```swift
 let client = Client(
     serverURL: try Servers.Server1.url(),
-    transport: URLSessionTransport(),
-    middlewares: [
-        BearerAuthMiddleware(token: yourJWTToken)
-    ]
+    transport: URLSessionTransport()
 )
 ```
 
@@ -21,4 +18,7 @@ let client = Client(
 ### Server URLs
 
 - ``Servers/Server1``
+
+### Deprecated
+
 - ``Servers/server1()``