V3 onion services: client-and-service side implementation

This is the ticket for tracking our progress implementing the client side of V3 onion service support in arti. We will need at least the following:

1. Descriptor Encoding

• Encoding/Decoding (est 24h) #683 (closed)
• Cryptography and Validation (est 24h) #684 (closed), #685 (closed)

2. Directory Support

• Descriptor outer layer decoding (1) (est 8h)
• Ring implementation (time period, SRV) (est 16h) #686 (closed), #687 (closed)
• Storage/Caching (est 16h) #689 (closed)

3. Circuits #688 (closed), #691 (closed)

• Introduction (ESTABLISH_INTRO, INTRO_ESTABLISHED and INTRODUCE) (est 8h)
• Rendezvous (ESTABLISH_RENDEZVOUS, RENDEZVOUS, RENDEZVOUS_ESTABLISHED) (est 8h)
• HSDir (upload and fetch) (est 12h)

4. Client #690 (closed), #692 (closed), #693 (closed), #694 (closed)

• Introduction (INTRODUCE, INTRODUCE_ACK) (est 20h)
• Rendezvous circuit crypto (ntor handshake) (est 30h)
• Descriptor fetch/cache (est 30h) #689 (closed)
• HSDir cache (est 30h)
• Intro point state cache (est 24h) #695 (closed)
• Client authorization (est 36h) #696 (closed)
• Rate limiting
  • Introduction attempts per intro point

5. Service #698 (closed)

• Ephemeral (ADD/DEL_ONION) #700 (closed)
• Cryptography (key blinding and derivation) (est 8h more; mostly done)
• Configuration (est 24h) #699 (closed)
• Building descriptor (with client auth) (est 36h)
• Events logic (est 30h) #697 (closed)
  • Establishing intro points
  • Expiring/Rotating intro points
  • Descriptor rotation
• Responding to an INTRODUCTION (establish rendezvous) (est 16h) #697 (closed)
• Rate limiting
  • Introduction circuit retries (in C: get_max_intro_circ_per_period()) #723 (closed)