simplify builider api generics

[ltitanb]

• Simplify the BuilderApi and BuilderApiState traits
• Add a working example on how to use the Api in examples/status_api

fix issues from #69

[fbrv]

gtg, we should a bit more descriptive with generics name. Ie,

struct PbsState<S> {
  data: S
}

could be renamed as

puct struct PbsState<EData> {
  data: EData,
}

In order to make the code a easier to read in case someone miss bits of documentation

[David-Petrov]

Everything looks fine!

Regarding what Fabio said about generic type parameters' names, I'm also a fan of more descriptive (whole-word even) type names, but this can easily become cumbersome, too. For me, right now a satisfactory solution would be to just describe type parameters in the rustdoc of the struct/trait that introduces them.

[ltitanb]

Sounds good, will merge as of now and add this note in the open issue we have for internal docs