feat: implement NIP-98 HTTP auth for gRPC

Add comprehensive NIP-98 authentication support following the standard
gRPC credentials.PerRPCCredentials pattern.

Client-side:
- NostrCredentials implements PerRPCCredentials interface
- Automatically signs each request with kind 27235 event
- Drop-in replacement for OAuth2/JWT in gRPC clients

Server-side:
- Unary and stream interceptors for validation
- Extracts and validates NIP-98 events from Authorization headers
- Configurable options (timestamp window, whitelists, skip methods)
- Adds authenticated pubkey to request context

Security features:
- Replay protection via timestamp validation
- Optional payload hash verification
- Signature verification using schnorr
- TLS requirement option

Includes comprehensive test coverage and detailed README with
usage examples and security considerations.

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