feat: add Xendit integration

[pranavrajs]

Adds the Xendit integration (Payments & billing). Look up transactions, payments, customers, and subscription plans in Xendit.

Tools

• Find Transactions by Reference (find_transactions, GET)
• Get Transaction (get_transaction, GET)
• Get Payment Request (get_payment_request, GET)
• Get Payment (get_payment, GET)
• Find Customer by Reference (find_customer_by_reference, GET)
• Get Subscription Plan (get_subscription_plan, GET)
• List Subscription Cycles (list_subscription_cycles, GET)

Testing

• The manifest passes the validator ("valid": true).
• Every template was rendered locally with Liquid 5.4.0 against sample responses built from Xendit's API docs.
• Not run against the live API. No credentials were available, so tool-tester.rb has not been run with a real key. This needs doing before merge.
• No logo is included.

Review notes

• No email search: lookups go by the merchant's reference ID or Xendit IDs.
• No api-version header is sent, because Xendit versions each endpoint family separately; each endpoint runs on its default version.

The root README.md row and .env.example entry for this integration are in a separate PR, so the per-integration PRs do not conflict with each other.

[pranavrajs]

Closing: not widely used in the catalog's target markets (US, India, Europe, Brazil, Argentina).