enable custom path of sse server

[elsejj]

Motivation and Context

For an access server that might proxy multiple MCP servers, although it's possible to customize the path for each MCP using Server, it's too cumbersome compared to FastMCP. Consider adding a custom path setting option to FastMCP.

How Has This Been Tested?

yes

Breaking Changes

No, default value is as same as before.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed

Additional context

# setup a sse server on custom path `/mcp1/sse`
FastMCP("Demo", 
  sse_path = "/mcp1/sse",
  message_path="/mcp1/message/"
)

# setup a sse server on default path `/sse`
FastMCP("Demo")