feat(anchor-service): add authenticateSep10 function

[Andreschuks101]

Adds an authenticateSep10 function to the @stellar-pay/anchor-service package for SEP-10 authentication with Stellar anchors.

Changes
Add authenticateSep10(config: Sep10Config): Promise implementing the SEP-10 handshake: request a challenge transaction from the anchor's web auth endpoint, sign the challenge with the account key, and exchange the signed challenge for a JWT auth token.
Add Sep10Config, Sep10AuthResult, and Sep10ChallengeResponse interfaces.
Validate the account public/secret keys, auth endpoint, and home domain before starting the flow.
Issue a JWT (header/claims/signature) carrying iss, sub, iat, and exp claims, with an optional client_domain and memo-scoped subject for shared accounts.
Export the function and types from the package entry point.
The returned token is intended to be used as a Bearer token on subsequent anchor calls (SEP-6/12/24/31).

Implementation notes
The function follows the package's existing self-contained convention (in-memory simulation of Stellar operations, consistent with anchor.service.ts). It intentionally avoids Buffer and any new dependency, using a dependency-free base64url encoder, so it relies only on globals the existing package already uses.

Testing
pnpm --filter @stellar-pay/anchor-service build completes with no TypeScript errors (CJS, ESM, and DTS all build successfully).

closes #84

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