[RestServer] API for user requests (sign up, vc request, storage)

[yiyione]

This PR is a part of feature Group & VC request management for user and admin #4949

APIs for user request:

The request struct example:

VC request
{
    "type": "vc",
    "id": "fdbb95de-eb61-48e7-aba5-e9811de14049",
    "username": "yiyione",
    "message": "Need permission for vc1.",
    "state": "new",
    "requestTime": "2020-10-12T03:22:04.407Z",
    "vc": [ "vc1" ]
}

User request
{
    "type": "user",
    "id": "b3610b79-9273-468c-8cba-615665ed321a",
    "username": null,
    "message": "test user sign up in basic mode",
    "state": "new",
    "requestTime": "2020-10-12T03:22:04.407Z",
    "user": {
        "username": "testuser",
        "email": "[email protected]",
        "passwordHash": "51f25c2a2c265e461083ca8492c024bf8dc02d14ae306f5f07dde8388a3cfb8a174c77cda596bb48292e4cb70328b84ceeb200e59eb0d6aa4d17f195bfb4567c"
    }
}

POST /api/v2/requests/user

Create a user request, it's the API for user sign up (without token check)

POST /api/v2/requests/:requestType

Create a request, for vc/storage request

GET /api/v2/requests/:requestType

Get requests, the requestType should be one of [ user, vc, storage ].

PUT /api/v2/requests/:requestId

Update a request state (new, approved or rejected), the requestId should be an uuid.
Admin only.

DELETE /api/v2/requests/:requestId

Delete a request
Admin only.

[coveralls]

Coverage decreased (-0.1%) to 33.894% when pulling 40f63a4 on yiyi/restserver_user_vc_request into b8c57ad on master.

[abuccts]

the name is strange, sounds like no http request received

[yiyione]

Change to NoUserRequestError.

[abuccts]

update api docs accordingly?

[yiyione]

Added.