Checkout: no way to add a merchant's terms and conditions checkbox

A merchant needs buyers to accept their Terms of Service at checkout for compliance. They do not have a sign-up page yet.

Tested custom_fields with field_type: 'boolean' and required: true:
https://test.checkout.dodopayments.com/session/cks_0NpLCLvsRsg39wolF1Zs2

1. No clickable links in labels. Markdown, plain URLs and HTML all show as plain text.
2. "Required" does not force a tick. A boolean is a Yes/No toggle, so the buyer can leave it unticked and pay. It is recorded as tos_accepted: "false".
3. Checkout only shows Dodo's own Privacy and Buyer Terms. There is no option for the merchant's.

For the purchase the buyer is buying from Dodo as MoR, and our Buyer Terms on checkout covers that. Their own Terms of Service is about using their product (licence, usage, data), which sits between them and their user, so it is better collected on their side.

Slack thread: https://dodopayments.slack.com/archives/C0C62C5B34L/p1791520510777839