Testnet faucet improvements

[DOBEN]

Task description

Sub-tasks

• Enable TLS support: Use credentials.createSsl() and the node with https connection (https://grpc.testnet.concordium.com:20000) in the Readme as an example for the env setting:
  https://github.com/Concordium/concordium-dapp-examples/blob/main/testnet-faucet/src/lib/createGPRCClient.ts#L8
• Move usage limit check to backend - https://github.com/Concordium/concordium-dapp-examples/blob/main/testnet-faucet/src/pages/index.tsx#L127. The usage limit should be checked as part of this API call before sending the funds:
  https://github.com/Concordium/concordium-dapp-examples/blob/main/testnet-faucet/src/pages/api/validateAndClaim.ts
• Enable support for hours instead of days: NEXT_PUBLIC_USAGE_LIMIT_IN_DAYS=1 env should work with e.g .0.25 for 6 hours.
• Add server-side Turnstile token verification:
  https://developers.cloudflare.com/turnstile/get-started/server-side-validation/

After MVP:

• Add database to backend and track nonce of senderAccount. Use the nonce of senderAccount in a locking manner to allow more than one person to claim from the faucet at the same time.
• Dry running the transaction should be implemented at the backend, to only send transactions if they are valid. One possible cause of a failing transaction is if the receiver account passed from the front end is not valid (does not exist on the chain).