[auction-sponsored-transactions] Additional improvements

[DOBEN]

Tasks

• Simplify ccd-js-gen integration with the browser wallet once ccd-js-gen: Generate functions for generating web wallet parameters concordium-node-sdk-js#324 is merged and a new release of the ccd-js-gen tool is available. Remove hardcoded schemas from the front end.
• Use ContractClient from the Rust SDK to simplify the interactions with the contract at the backend Sponsored transaction enabled auction #32 (comment)
• Instead of a hardcoded node connection at the front-end to construct the grpc client, use the node connection provided from the connected wallet.
• Consolidate steps 4 and 5, so that no signerAccount has to be inputted in the input field anymore. The signerAccount can be stored in a useState hook at the point the signature is generated. This will minimize user error.