serde diff enum representation

[carlocorradini]

Allow for customization of the generated diff enum serde representation.
Currently, each diff is serialized/deserialized using the default Externally tagged enum representation, with the key being the field's name and the value being the updated value. This is difficult to parse effectively and safely in TypeScript since the object lacks a common discriminant field.
By allowing us to change the enum representation, we gain a lot of versatility because we can select between:

• Externally tagged (default)

  #[derive(Serialize, Deserialize, Difference)]
  struct Example { }

• Internally tagged

  #[derive(Serialize, Deserialize, Difference)]
  #[difference(serde(tag = "type"))]
  struct Example { }

• Adjacently tagged

  #[derive(Serialize, Deserialize, Difference)]
  #[difference(serde(tag = "t", content = "c"))]
  struct Example { }

• Untagged

  #[derive(Serialize, Deserialize, Difference)]
  #[difference(serde(untagged))]
  struct Example { }

[knickish]

I think this is reasonable. I would prefer to solve this by allowing arbitrary pass through to the derived enum type

[carlocorradini]

Would you agree with something like the following?

#[derive(Serialize, Deserialize, Difference)]
#[difference(attr(
    #[derive(tag = "field", content = "value")]
))]
pub struct ProjectMeta { ... }