Add agreement_terms field in the DataDeal

[youngjoon-lee]

To show providers details agreement terms, we need to add a new field, agreement_terms, which is a string array of URLs, which is a array of AgreementTerm. We'll expect that data consumers write terms in Markdown, but the chain doesn't need to validate the contents/format of a description in the AgreementTerm.

Deal

{
   "data_schema": [...],
   "agreement_terms": [
          {
                "id": "term-1",
                "required": true,
                "description": "blah blah (Markdown is recommended)"
          }
    ]
}

Consent

{
    "certificate": {
         "deal_id": 1,
    },
   "deal_id": 1,
   "agreements": [
         {
              "term_id:": "term-1",
              "agreement": true
         },
         {
              "term_id:": "term-2",
              "agreement": false
         },
   ]
}

The chain has to reject MsgSubmitConsent which doesn't contain agreements for required terms.

This field shouldn't be modified (added/edited/removed) after the deal is created. Especially, it shouldn't be modified after at least one data is provided with the deal.

[youngjoon-lee]

Discussed with @tjyoon0324 offline. Let's go with the spec described above, which is simple enough for covering many unexpected cases.