MCP External Library Callback Support

[xorroproprop]

There currently doesn't seem to be a way for handling potential callbacks from 3rd party libraries. I found that using thread locking allowed the callback function to execute. However, I think there could be a more "graceful" way of handling this.

`python
from foo import func
from mcp.server.fastmcp import FastMCP

def cb_func()
print(bar)

@mcp.tool(name="callback_foo")
def callback():
foo = func(file_path, cb_func)
`