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]

@erdoganishe Thanks for the updates. The PR is nearly ready to merge. As this is your first PR, I am being a bit more nit-picky than usual to draw your attention to commit practices 🙂:

nit: Git log entries with Changelog-* tags generate the release changelog from PRs merged between releases. While multiple changelog entries per PR are fine, they should represent independent changes. For instance, if one commit adds a field and another removes an RPC, separate entries make sense. However, this PR makes a single logical change from a release perspective: adding a description field across multiple RPCs.

So rather than having multiple entries, we can include a single changelog entry in any one commit, like: "Changelog-Added: Add description field to listoffers, encodeoffer, decodeoffer, and the sql plugin’s offers table."

---

nit: We already have a commit titled doc: Added doc schemas with description and update proto that updates the protobufs. Could we either merge the changes from the later commit (grpc: regenerate Python protobufs after rebase) into that commit, or split the original commit into two logical ones like: doc: Added doc schemas with description and grpc: Update protobufs with description field.

---

nit: I initially added my changes into a separate commit (lightningd: Updated offer methods) to make the changes clearer for your review. However, this commit isn’t meaningful for future references and should be merged with lightningd: add description field to offer related responces.