feat: myaccount API support

[rmad17]

📋 Changes

This PR carves the My Account API slice out of #116, which combined Passkey, My Account, and DPoP into one large diff. It's the second of three sequential branches (Passkey → My Account → DPoP), split for merge-conflict legibility.

Refer to #116 for the original review discussion and feedback — the code here was reviewed there; this PR only re-sequences it.

Features

• My Account API — Authentication Methods & Factors (MyAccountClient): get_factors, list_authentication_methods, get_authentication_method, delete_authentication_method, update_authentication_method, enroll_authentication_method, and verify_authentication_method. Lets
  a logged-in user manage their own enrolled authentication methods (passkey, email, phone, TOTP, push, recovery-code, password) against the My Account API,
  scoped by fine-grained scopes like read:me:factors.
• Enrollment and verification are generic across method types — the type is set via EnrollAuthenticationMethodRequest.type, not a dedicated method per type.

API Changes

• New public methods on MyAccountClient: get_factors, list_authentication_methods, get_authentication_method, delete_authentication_method, update_authentication_method,
  enroll_authentication_method, verify_authentication_method.
• New public models: Factor, GetFactorsResponse, AuthenticationMethod, ListAuthenticationMethodsResponse, EnrollAuthenticationMethodRequest, EnrollmentChallengeResponse, VerifyAuthenticationMethodRequest, UpdateAuthenticationMethodRequest, plus type aliases EnrollmentType
  and PreferredAuthMethod.
• No new error types — these methods raise the existing MyAccountApiError/ ApiError / MissingRequiredArgumentError, consistent with the rest of MyAccountClient.
• New doc: examples/MyAccountAuthenticationMethods.md
  (prerequisites, obtaining a scoped token, all six operations, error handling). README gains a linking section; Passkeys.md gains a cross-reference note pointing here for passkey management (as opposed to passkey sign-in).

[kishore7snehil]

LGTM!