Add decorator to simplify creating JSON REST endpoints

[marcusschiesser]

Adds the decorator json_handler to simplify creating JSON REST endpoints. It automatically adds an exception handler and injects the following parameters to the handle method:

1. request, which is json.loads(in_string)
2. query, which contains the URL parameters as a dict
3. payload, which contains the payload as dict (remember to set passPayload = true in restmap.conf)

Here's an example for a GET handler with URL and path parameters:

from splunk.persistconn.application import PersistentServerConnectionApplication
from splunklib.customrest import json_handler

class GetHandler(PersistentServerConnectionApplication):
    def __init__(self, _command_line, _command_arg):
        PersistentServerConnectionApplication.__init__(self)

    @json_handler
    def handle(self, request, query, **kwargs):
        path_param = request['path_info']
        url_param1 = query.get('param1', 1)
        url_param2 = query.get('param2', 1)
        return {'payload': {'path_param': path_param, 'url_param1': url_param1, 'url_param2': url_param2},
                'status': 200
                }

(Called with something like: http://localhost:8000/en-US/splunkd/__raw/servicesNS/-/{app-name}/myapi/333?param1=4)

And an example for a POST handler (with passPayload = true) which processes a JSON payload:

from splunk.persistconn.application import PersistentServerConnectionApplication
from splunklib.customrest import json_handler

class PostHandler(PersistentServerConnectionApplication):
    def __init__(self, _command_line, _command_arg):
        PersistentServerConnectionApplication.__init__(self)

    @json_handler
    def handle(self, payload, **_kwargs):
        data = payload['flag']
        
        return {'payload': {'data': data},
                'status': 200