feat(http): add transport extension hooks

[patlux]

Motivation

tauri-plugin-http currently creates an internal reqwest::ClientBuilder for every request without exposing a native customization point. Applications that need transport-level policies or custom TLS backends must therefore replace the fetch-compatible plugin surface with custom commands.

This change adds trusted native transport extensions while preserving the existing JavaScript API and default init() behavior.

Design

• tauri_plugin_http::Builder<R> registers extensions in deterministic order.
• HttpTransportExtension<R> exposes fallible setup, request validation, final reqwest::ClientBuilder configuration, and transport-error mapping.
• RequestContext provides owned request metadata without exposing request bodies.
• ExtensionError forwards extension-owned structured JSON through IPC while existing plugin errors remain strings.
• Stock timeout, redirect, proxy, cookie, and feature-gated TLS settings are applied before transport extensions configure the final client.

This provides a general foundation for native transport concerns such as custom TLS verification, mTLS, request policy enforcement, and transport diagnostics. SPKI pinning from #3414 can be implemented as a separate extension on top of this API.

Follow-up implementation: #3539.