Allow Deprecation of Optional Arguments

[dice14u]

When evolving an api there are often times when a new argument may take the place of an older one. In doing so it would be beneficial to set the old argument as optional and mark it deprecated.

example

type Zoo {
  getAnimals(
    "Example of some originally mandatory field"
     filter: String!
   ): [Animal]
}

may find it is not sufficient for the api and then evolve to

type Zoo {
  getAnimals(
   "Field can no longer be mandatory because it is going to disappear"
    filter: String @deprecated(reason = "Use specific taxa filters removal on 1/1/2020")
    phylum: String
    kingdom: String
    order: String
  ): [Animal]
}

In this example the implementer does not want to have to support parsing the filter string argument forever and so is marking it deprecated. Also because the field was originally a primitive "String" the input type itself cannot be deprecated.

[IvanGoncharov]

@dice14u Thanks for detail description 👍
This proposal already tracked as #525 and you are welcome to join the discussion there.