Post OptionalData improvements

[RiccardoM]

Currently inside the x/posts module we have defined two types to handle the optional data that's associated with a post:

• OptionalDataEntry that represents a single optional data, and
• OptionalData that represents a slice of optional data

In my opinion, these very similar names might be confusing for external developers. Also, I think the current naming of OptionalData might be confusing as well. For this reason, I propose the following changes:

1. Remove the OptionalData type and use only the []OptionalDataEntry
2. Rename the OptionalDataEntry type to be Attribute
3. Rename the OptionalData field of a Post into AdditionalAttributes

This way, the final Post object would then be something like

type Post struct {
  AdditionalAttributes []Attribute
}

IMO, this would make it very easy for external developers to understand that they can insert additional things inside that array.