In SERVER-63870, some code was duplicated between setClusterParameter and _configsvrSetClusterParameter. This primarily entails the logic to transform the BSON in the command argument into the on-disk format and validate the server parameter. This code can be modularized into a single method that is shared by both commands, and we must also create a struct to refer the _id and clusterParameterTime fields instead of hardcoding the name directly.

Additionally, there may be opportunity to move some of the checks on the request itself into a `validateRequest()` method. This needs some more investigation.