iOS B2C Crypto Wallets client

[susie-stytch]

Linear Ticket: SDK-1469

Changes:

Adds stytch.cryptoWallets with the following methods:

• authenticateStart
• authenticate

Notes:

Checklist:

• I have verified that this change works in the relevant demo app, or N/A
• I have added or updated any tests relevant to this change, or N/A
• I have updated any relevant README files for this change, or N/A

[jhaven-stytch]

This seems good, I just wanna confirm we're on the same page w/r/t the signing of challenges: since iOS and Android don't have the same concept of a wallet manager (or whatever it's called) like we do in the JS SDK, our SDK has no responsibility to perform the signing flow, right? We're just an intermediary between passing the challenge back to the developer, and then it's their responsibility to sign the challenge and return it to us. Thats my understanding, and what appears to be happening here, just wanna make sure!