#5266 Gateway: Validate changes to allowedSlippage

• Improve validation for /config/update when configPath ends with allowedSlippage.
• Allow any value equal to 0.0 and less than 1.0. Lower or higher values are rejected.
• Allow the user to provide the new value in the following formats: 0.5, "1/100", "0.25".
• Reorganize config route code

Link: https://github.com/hummingbot/hummingbot/pull/5266