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 中搜尋 McpAsyncClient 和 McpSyncClient，並檢視現有的 GitHub issue，尋找重疊的請求。確定對 SDK 和第三方實作必須一致公開的用戶端行為；完成表示兩個用戶端都具備穩定的介面設計，支援在不修改原始碼的情況下進行自訂，並保留其相關功能。