Support to return data when signature is saved

[martindsouza]

Background: See #13 for Page Items to Submit and the session state option.

The use-case I have is the following:

• Save the signature into a collection
• When saving into a collection I want the seq_id and store this in a page item (let's sage p1_seq_id
• Use that p1_seq_is as part of my regular page submit process which I'll then get the blob from the apex_collection

In order to do this need to pass a data object back in the PL/SQL code. Ex:

-- ...
apex_json.open_object;
apex_json.write('seqId', l_seq_id); 
apex_json.close_object; 
-- ...

[martindsouza]

Also updated help text for PLSQL Code to include at end:

  -- #15: Option can return data back to Javascript (available in this.data)
  apex_json.open_object;
  apex_json.write('someId', 123); 
  apex_json.close_object; 

[martindsouza]

@Dani3lSun Note: I have the code ready to do a PR but need #14 accepted first.