Refactor Client Class to Follow [finData, request] OPRF Convention

[sublimator]

Description

The current implementation of the Client class is designed to handle a single request at a time. This design can be surprising/confusing and does not align with the convention used in the OPRF setup, which returns a tuple [finData, request].

Proposed Solution

Two possible ways to approach this refactoring:

1. Rename Client to TokenRequestContext to better convey the per-request state and update its methods accordingly.
2. Modify the createTokenRequest method to return a tuple [finData, request] instead of a single TokenRequest object.

It's proposed to refactor the Client class to adhere to the OPRF convention.

export class TokenRequestContext {
    private finData?: {
        pkIssuer: CryptoKey;
        tokenInput: Uint8Array;
        authInput: AuthenticatorInput;
        inv: Uint8Array;
    };

    async createRequest(
        tokChl: TokenChallenge,
        issuerPublicKey: Uint8Array,
    ): Promise<TokenRequest> {

or

interface Client2State {
    authInput: AuthenticatorInput;
    finData: FinalizeData;
}

export class Client2 {
    vClient: Client<ModeVoprf>;

    constructor(private issuerPublicKey: Uint8Array) {
        this.vClient = MODE.makeClient(issuerPublicKey);
    }

    async createTokenRequest(
        tokChl: TokenChallenge,
    ): Promise<[finData: Client2State, request: TokenRequest2]> {