Parameter Dynamic Required vs Optional Validation

First thanks for the sweet framework, my team and I are big fans of grape. And perhaps this a good opportunity to contribute back.

I was however unable to find any additional information related to the ability to change params required/optional state at runtime/request time.

The use case is that the API based off some values in the request can dynamically enforce optional vs required behavior. Namely a request that looks something like this

{
"required" : "param1,param2,param3",
...
}

As far as I can tell this is currently not possible? Maybe hooking into the pre_validation block?

Could this be added as a validation feature? Perhaps a new type called dynamic that has some matching/parsing convention that can be user defined

Thanks!

Start by examining the framework's pre_validation hook and existing parameter-validation behavior. Define how request-time required and optional states should be expressed and verified, including the proposed dynamic validation type; no file or test is named in the issue.