Implement a serde Deserializer for `ConstantValue` + `Vec<NameValuePair<DirectiveArgumentName, ConstantValue>>` and derive

[rbalicki2]

• We should implement a serde deserializer for ConstantValue, Vec<NameValuePair<DirectiveArgumentName, ConstantValue>>.
• We get Vec<NameValuePair<DirectiveArgumentName, ConstantValue>> as part of parsing schema directives
• So, in particular, we get a Vec<...> from parsing

  @exposeField(
  field: "set_pet_tagline"
  path: "pet"
  field_map: [{ from: "id", to: "input.id" }]
  )

• This gets turned into a MagicMutationFieldInfo
• We should use a serde derive to replace validate_magic_mutation_directive

[rbalicki2]

@edmondop