CDB3-347 Registry field 'relatedDocument' can be used for non-digitalSignature docs

Summary: field can be used for non-digitalSignature docs

STR:

1. Upload doc to tocument service, for example with type auctionProtocol
2. Go to /api/registry/objects/:obj_id/documents?acc_token= endpoint
3. Try to upload doc with next body:

{
    "token": "",
    "title": {
        "uk_UA": "Назва",
        "en_US": "Title"
    },
    "description": {
        "uk_UA": "Опис",
        "en_US": "Description"
    },
    "documentOf": "leaseRequest",
    "documentType": "auctionProtocol",
    "index": 33,
    "format": "string",
    "language": 0,
    "relatedDocument": ""
}

Actual result: doc uploaded with 'relatedDocument' field

Expected result: https://prnt.sc/w2852f plz fix for all docs in Registry