Support Custom ChallengeGenerator

[zunda-pixel]

I want to use secure challenge value.

let webAuthn = WebAuthnManager(
  configuration: .init(
    relyingPartyID: "RELYING_PARTY_ID",
    relyingPartyName: "RELYING_PARTY_NAME",
    relyingPartyOrigin: "RELYING_PARTY_ORIGIN"
  ),
  challengeGenerator: .init {
    SymmetricKey(size: .bits256).withUnsafeBytes { Array($0) } // 32 bytes
  }
)