[Quickstarts] Add env vars for configuring payments subgraph URLS

[MantisClone]

Problem

The Quickstarts suffer from the same rate-limited payments subgraph URLs that we fixed in RequestNetwork/invoicing-template#41

Solution

Update the Quickstarts to use the latest RN SDK version which includes the Alchemy Payment Subgraph URLs by default.

Definition of Done

Please link the PRs to this issue.

• https://github.com/RequestNetwork/quickstart-node-js
• https://github.com/RequestNetwork/quickstart-browser-create-a-request
• https://github.com/RequestNetwork/quickstart-browser-pay-a-request
• https://github.com/RequestNetwork/quickstart-browser-retrieve-a-request
• https://codesandbox.io/p/devbox/silly-fermi-shffng (create, used by docs)
• https://codesandbox.io/p/devbox/adoring-bohr-dn7kcf (pay, used by docs)
• https://codesandbox.io/p/devbox/upbeat-panka-mqrjqy (retrieve, used by docs)

[MantisClone]

Do a stable release before this.