Extract McpAsyncClient and McpSyncClient as an interface

Please do a quick search on GitHub issues first, the feature you are about to request might have already been requested.

Expected Behavior

McpAsyncClient and McpSyncClient a interfaces

Current Behavior

classes

Context

We are implementing a relatively complex client to support long-running tool calls and machine restarts. However, the current client implementation makes it difficult to customize without modifying the source code.

We hope to introduce an interface that, while providing the relevant functionality, also provides consistent behavior from the Java SDK to second- and third-party clients.

首先在 Java SDK 中搜索 McpAsyncClient 和 McpSyncClient，并查看现有的 GitHub issue，寻找重叠的请求。确定对于 SDK 和第三方实现必须一致公开的客户端行为；完成意味着两个客户端都具有稳定的接口设计，支持在不修改源代码的情况下进行定制，并保留其相关功能。