Add authentication support for MCP servers

[elijahdev0]

This PR implements authentication support for FastAPI-MCP as discussed in issue tadata-org#10.

Features added:

• Authentication module with support for:
  • Bearer token authentication
  • API key authentication (in header or query)
  • Custom authentication functions
• Added auth_config parameter to add_mcp_server and mount_mcp_server functions
• Authentication is applied to both the SSE connection endpoint and message endpoint
• Updated documentation in README
• Added examples:
  • authenticated_server.py - JWT authentication example
  • simple_bearer_auth.py - Simple bearer token example

This implementation follows the MCP authentication specification, which recommends using bearer tokens in the Authorization header.

How to use:

from fastapi import FastAPI
from fastapi_mcp import add_mcp_server, AuthConfig

app = FastAPI()

# Configure authentication
auth_config = AuthConfig(
    enabled=True,
    bearer_token="your-secret-token"
)

# Add MCP server with authentication
mcp_server = add_mcp_server(
    app,
    mount_path="/mcp",
    auth_config=auth_config
)

For more complex authentication scenarios, custom authentication functions are supported.

Fixes tadata-org#10

[shahar4499]

@elijahdev0 Hey, thank you! Looks promising. I will review this PR, but you need to open it in the original repo for it to be mergeable. Right now it's open only in your fork :)