Add parameter validation to Ktor server generator

[atollk]

see original discussion here: #280 (comment)

#280 implemented Ktor server generation but compared to the Spring and Micronaut generators, it is missing validation checks on the format of request parameters, e.g. the maximal allowed length of a string. It would be nice to have all generators be on the same capability level.

Since Ktor does not offer annotations for automatic checks like Spring and Micronaut do, a different solution needs to be found.

cc @ulrikandersen