Add a Guide to override payment subgraph URLs when instantiating RequestNetwork instance

[MantisClone]

Problem

Some App Builders may wish to override the payment subgraph URLs so they can use their own paid subgraph URLs instead of relying on the rate-limited URLs provided by the Request Network SDK by default. We should add a Guide that explains how to do so.

Example

https://github.com/RequestNetwork/invoicing-template/blob/01e44755b274d17c0718ca03f077d68a9fe8baec/utils/initializeRN.ts#L17-L59

      paymentOptions: {
        getSubgraphClient: (chain: string) => {
          // Ternary because cannot dynamically access environment variables in the browser
          const paymentsSubgraphUrl =
          chain === "arbitrum-one"
            ? process.env.NEXT_PUBLIC_PAYMENTS_SUBGRAPH_URL_ARBITRUM_ONE || "https://subgraph.satsuma-prod.com/e2e4905ab7c8/request-network--434873/request-payments-arbitrum-one/api"
            : chain === "avalanche"
            ? process.env.NEXT_PUBLIC_PAYMENTS_SUBGRAPH_URL_AVALANCHE || "https://subgraph.satsuma-prod.com/e2e4905ab7c8/request-network--434873/request-payments-avalanche/api"
            : chain === "base"
            ? process.env.NEXT_PUBLIC_PAYMENTS_SUBGRAPH_URL_BASE || "https://subgraph.satsuma-prod.com/e2e4905ab7c8/request-network--434873/request-payments-base/api"
            : chain === "bsc"
            ? process.env.NEXT_PUBLIC_PAYMENTS_SUBGRAPH_URL_BSC || "https://subgraph.satsuma-prod.com/e2e4905ab7c8/request-network--434873/request-payments-bsc/api"
            : chain === "celo"
            ? process.env.NEXT_PUBLIC_PAYMENTS_SUBGRAPH_URL_CELO || "https://api.studio.thegraph.com/query/67444/request-payments-celo/version/latest"
            : chain === "core"
            ? process.env.NEXT_PUBLIC_PAYMENTS_SUBGRAPH_URL_CORE || "https://thegraph.coredao.org/subgraphs/name/requestnetwork/request-payments-core"
            : chain === "fantom"
            ? process.env.NEXT_PUBLIC_PAYMENTS_SUBGRAPH_URL_FANTOM || "https://subgraph.satsuma-prod.com/e2e4905ab7c8/request-network--434873/request-payments-fantom/api"
            : chain === "fuse"
            ? process.env.NEXT_PUBLIC_PAYMENTS_SUBGRAPH_URL_FUSE || "https://api.studio.thegraph.com/query/67444/request-payments-fuse/version/latest"
            : chain === "mainnet"
            ? process.env.NEXT_PUBLIC_PAYMENTS_SUBGRAPH_URL_MAINNET || "https://subgraph.satsuma-prod.com/e2e4905ab7c8/request-network--434873/request-payments-mainnet/api"
            : chain === "matic"
            ? process.env.NEXT_PUBLIC_PAYMENTS_SUBGRAPH_URL_MATIC || "https://subgraph.satsuma-prod.com/e2e4905ab7c8/request-network--434873/request-payments-matic/api"
            : chain === "moonbeam"
            ? process.env.NEXT_PUBLIC_PAYMENTS_SUBGRAPH_URL_MOONBEAM || "https://api.studio.thegraph.com/query/67444/request-payments-moonbeam/version/latest"
            : chain === "optimism"
            ? process.env.NEXT_PUBLIC_PAYMENTS_SUBGRAPH_URL_OPTIMISM || "https://subgraph.satsuma-prod.com/e2e4905ab7c8/request-network--434873/request-payments-optimism/api"
            : chain === "sepolia"
            ? process.env.NEXT_PUBLIC_PAYMENTS_SUBGRAPH_URL_SEPOLIA || "https://subgraph.satsuma-prod.com/e2e4905ab7c8/request-network--434873/request-payments-sepolia/api"
            : chain === "xdai"
            ? process.env.NEXT_PUBLIC_PAYMENTS_SUBGRAPH_URL_XDAI || "https://api.studio.thegraph.com/query/67444/request-payments-xdai/version/latest"
            : chain === "zksyncera"
            ? process.env.NEXT_PUBLIC_PAYMENTS_SUBGRAPH_URL_ZKSYNCERA || "https://subgraph.satsuma-prod.com/e2e4905ab7c8/request-network--434873/request-payments-zksyncera/api"
            : undefined;
          if (!paymentsSubgraphUrl) {
            throw new Error(
              `Cannot get subgraph client for unknown chain: ${chain}`
            );
          }
          return getTheGraphClient(chain, paymentsSubgraphUrl);
        },
      },