Add sumo_rest_doc:from_ctx/1 callback

[elbrujohalcon]

The idea is to give model implementors access to what currently is on sumo_rest handlers state.
The tasks are as follows:

• Abstract sr_entities_handler and sr_single_entity_handler states to a new ADT called sr_state:state(). Defined as follows:

-module(sr_state).

-opaque state() ::
  #{opts := sumo_rest:options()
   , id => binary()
   , entity => sumo:user_doc()
   , module := module()
   , user_opts := map()
   }.
-export_type([state/0]).

%% constructor, getters and setters
-export([new/2, id/1, id/2, entity/1, entity/2, module/1, opts/1]).
%% functions to work with user opts
-export([set/3, retrieve/3, remove/2]).
-spec set(Key::term(), Value::term(), state()) -> state().
-spec retrieve(Key::term(), state(), Default::T) -> T.
-spec remove(Key::term(), state()) -> term().

• Create a new ADT for request data, defined as follows.

-module(sr_request).
-opaque req() ::
  #{body => sr_json:json()
   , headers := [{binary(), binary()}]
   , path := binary()
   , bindings := #{atom() => binary()}
   }.
-export_type [req/0].

-export([new/1]).

-spec new(cowboy_req:req()) -> req().

• Add a new type and callback to sumo_rest_doc, as follows:

-type context() :: #{req := sr_request:req(), state := sr_state:state()}.
-callback from_ctx(context()) -> sumo:user_doc().
-optional_callbacks([from_ctx/1]).

• Turn from_json/1 into an optional callback

• Change the handlers so that, whenever Module:from_json/1 is called now, before doing that, if Module:from_ctx/1 is defined, they use that one.

• Update the docs and examples