Make reusable wrapper for `SSRFProtect.validate()` that reads Constance settings

[jnm]

it'd be nice to have the constance setting parsing and string manipulation stuff be in some kind of central utility, so that people could perform a validation with one function call

(instead of doing all that settings parsing and string manipulation each time we call SSRFProtect.validate())

Originally posted by @jnm in #5041 (comment)