Update iOS User headless client

[susie-stytch]

Linear Ticket: SDK-1471

Changes:

Adds the following methods to the user management client:

• onChange
• deleteFactor for TOTP and OAuth registrations

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]

Is it possible to emit the user in the onChange method, instead of just the User.ID? It looks like that's what we're doing in JS land, and would like to keep them the same, if possible

[jhaven-stytch]

LGTM