Extract McpAsyncClient and McpSyncClient as an interface

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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.