feat: add env vars for configuring payments subgraph URLs

[MantisClone]

Problem

The rate-limit on the free subgraph URLs from Subgraph Studio is lower than expected. The net effect is that Request Payment (https://invoicing.request.network/) will often (but not always) fail to detect that a request is paid.

• When making a payment, the UI will often tell you the payment failed, even though the transaction went through.
• When displaying the dashboard, paid requests would often display "created" status instead of the expected "paid" status.

Bandaid Solution

Get paid URLs from Subgraph Studio.

Changes

• feat: add NEXT_PUBLIC_PAYMENT_SUBGRAPH_URL_... env vars and getSubgraphClient() function for configuring payments subgraph URLs

Screenshots

This screenshot shows the "paid" status indicator on the invoice details drawer

This screenshot shows the "paid" status indicator on the invoice dashboard

Context

• The Graph Hosted Service ceased operation last night. This was expected.
• We already migrated to the free, rate-limited subgraph URLs provided by Subgraph Studio in:
  • https://github.com/RequestNetwork/private-issues/pull/77
  • chore(thegraph): migrate to TheGraph Studio requestNetwork#1402
  • chore(thegraph): fix xdai subgraph URL requestNetwork#1403
  • chore(thegraph): fix mainnet subgraph URL requestNetwork#1404

[MantisClone]

Merging without review to fix production Request Payments (https://invoicing.request.network)