lightningd: add description field to offer related responces

[erdoganishe]

Fixes #8554.

Add description field for offer related responces. Examples:

% lightning-cli --network=regtest --lightning-dir=/tmp/cln1 offer 100msat "Coffee payment" "" myoffer
{
   "offer_id": "41ebdcd9d7b72e7b6e4294ab6fec93c8e99c4fe1df7b3503386b64aeefc233f0",
   "active": true,
   "single_use": false,
   "bolt12": "lno1qgsqvgnwgcg35z6ee2h3yczraddm72xrfua9uve2rlrm9deu7xyfzrcgq9jq5rjrdanxvet9ypcxz7tdv4h8gysqzcss8zeszk6dq87kufv7nswc9l22kfsy3x9ckuxavvle8q27gf5k2j8u",
   "description": "Coffee payment",
   "used": false,
   "label": "myoffer",
   "created": false
}

% lightning-cli --network=regtest --lightning-dir=/tmp/cln1 listoffers
{
   "offers": [
      {
         "offer_id": "41ebdcd9d7b72e7b6e4294ab6fec93c8e99c4fe1df7b3503386b64aeefc233f0",
         "active": true,
         "single_use": false,
         "bolt12": "lno1qgsqvgnwgcg35z6ee2h3yczraddm72xrfua9uve2rlrm9deu7xyfzrcgq9jq5rjrdanxvet9ypcxz7tdv4h8gysqzcss8zeszk6dq87kufv7nswc9l22kfsy3x9ckuxavvle8q27gf5k2j8u",
         "description": "Coffee payment",
         "used": false,
         "label": "myoffer"
      }
   ]
}

[ShahanaFarooqui]

@rustyrussell Currently, listsendpays and sql -k query="SELECT * FROM sendpays;" do not return the Bolt11/Bolt12 description. In contrast, listinvoices and sql -k query="SELECT * FROM invoices;", and listoffers and sql -k query="SELECT * FROM offers;" (after this PR), will also show the description. Should we extend the same functionality to sendpays as well? If so, I will create a separate issue for that and keep this PR focused on offers only.

[ShahanaFarooqui]

Adding description here causes it to appear in the offer RPC response as well, which results in a msggen error because that field already exists in the offer request. We can rather add description separately in listoffers, disableoffer and enableoffer responses.

[ShahanaFarooqui]

A better approach is to use json_add_stringn, since it accepts an explicit length parameter and doesn’t require the string to be null-terminated. Using json_add_string will result in garbage data being appended after the description value.

[ShahanaFarooqui]

Type casting for description is not required.

[ShahanaFarooqui]

Whenever we add or remove an RPC, or modify any RPC fields, the documentation JSON schema must also be updated, as explained here, for Core Lightning to build successfully. It also auto-generates the new protos. In this case, we need to add the description field — along with its added version tag (v26.04) — to listoffers, enableoffer, and disableoffer.

[ShahanaFarooqui]

Writing a quick test for these new additions would be a good way to verify that the changes are working as expected. Currently, tests/test_plugin.py::test_sql is failing because the offers table in our SQL plugin now also expects the description field.