Treat all custom string challenges as UTF-8 strings

[MasterKale]

Describe the issue

generateRegistrationOptions() treats string values for the challenge option as ASCII strings when converting to bytes, while generateAuthenticationOptions() handles them like they're UTF-8 bytes:

https://github.com/MasterKale/SimpleWebAuthn/blob/a59634a1a9b0393622fb121fbe229132c01a2624/packages/server/src/registration/generateRegistrationOptions.ts#L160

https://github.com/MasterKale/SimpleWebAuthn/blob/a59634a1a9b0393622fb121fbe229132c01a2624/packages/server/src/authentication/generateAuthenticationOptions.ts#L49

Reproduction Steps

N/A

Expected behavior

string challenges should all be treated as UTF-8 strings for maximum flexibility for RP's that need it.

Code Samples + WebAuthn Options and Responses

N/A

Dependencies

N/A

SimpleWebAuthn Libraries

$ npm list --depth=0 | grep @simplewebauthn
├── @simplewebauthn/server@8.3.6
# ...

Additional context

N/A