Support for arbitrary data

[RiccardoM]

Context

As Desmos will be a public chain potentially used by many different applications as their backend storage, we might want to allow such applications to store arbitrary data inside posts.

Some examples can be the following.

• BigDipper might want to store a reference to the chain-id for which the message has been posted and/or the address of the original poster;
• applications having a local storage might want to store a locally-generated reference to a post in order to easily update its status when its uploaded to the chain.

Arbitrary data support proposal

In order to solve both issues in one single run, I suggest we implement the support for arbitrary data by adding a new field into the Post type.

For sake of simplicity I recommend we only support String -> String associations, and not any king of value type that might be used inappropriately.

With this, any developer can integrate whatever data he wants into the post itself, without having to add any overhead to the overall post structure.

Storage implementation

In order to implement such feature, we can change how Post objects are made, adding a new map[string]string field:

type Post struct {
    PostID            PostID            `json:"id"`
    ParentID          PostID            `json:"parent_id"`
    Message           string            `json:"message"`
    Created           sdk.Int           `json:"created"`
    LastEdited        sdk.Int           `json:"last_edited"`
    AllowsComments    bool              `json:"allows_comments"`
    ExternalReference string            `json:"external_reference"`
    Owner             sdk.AccAddress    `json:"owner"`

    OptionalData      map[string]string `json:"optional_data"` // Contains any kind of arbitrary data
}

I also think that we might want to move the existing ExternalReference field into such map to make the overall object more simple and having less fields leading thus to a smaller size on disk:

type Post struct {
    PostID            PostID            `json:"id"`
    ParentID          PostID            `json:"parent_id"`
    Message           string            `json:"message"`
    Created           sdk.Int           `json:"created"`
    LastEdited        sdk.Int           `json:"last_edited"`
    AllowsComments    bool              `json:"allows_comments"`
    Owner             sdk.AccAddress    `json:"owner"`

    OptionalData      map[string]string `json:"optional_data"` // Contains any kind of arbitrary data
}