Add Support for Amazon Bedrock Provider

Currently, the SDK includes built-in support for providers like OpenAI and Anthropic, but Bedrock — which unifies access to many high-quality foundation models — is not directly supported.
This enhancement would allow users of the SDK to leverage AWS’s scalable foundation models via a C++ interface alongside other providers.

**Motivation**
Amazon Bedrock is a fully managed AWS service that provides access to a wide range of foundation models from different providers via a single API. Bedrock supports many leading models across domains such as text, reasoning, coding, and multimodal tasks

**Proposed Solution**
Add a new provider module, e.g. ai/bedrock.h and supporting implementation similar to existing providers (openai, anthropic).
Use AWS SDK for C++ (Aws::BedrockRuntime::BedrockRuntimeClient) to handle API requests and authentication.
Expose key Bedrock features through the SDK’s unified interface (text completion, streaming, etc.).
Include examples/tests demonstrating a basic Bedrock request (e.g. text generation).

まず既存の openai および anthropic provider モジュールを確認し、次に提案されているエントリーポイント ai/bedrock.h と Aws::BedrockRuntime::BedrockRuntimeClient を調査します。テキスト補完、ストリーミング、認証、および統一インターフェースを Bedrock にどのようにマッピングするかを定義し、基本的なテキスト生成リクエストの例またはテストを追加します。

索引モデルが issue の本文から書いたものです。