Implement the v2 events endpoint and deprecate the existing getEvents #774

TL;DR

Following the getEvents product pass in #426, implement the redesigned v2 events endpoint and back-compat support for the existing v1 getEvents API.

Scope

In priority order:

• 1. Implement the v2 events endpoint per the direction settled in Revisit getEvents API #426.
  protocols/rpc: Add getEvents v2 wire types and request validation go-stellar-sdk#5971
  rpcv2: add the getEvents v2 event cursor and codec #914
  rpcv2: the getEvents v2 cross-chunk events pager #934
  rpcv2: the getEvents term budget knob, and the EventScope rename #939
  rpcv2: serve getEventsV2 #954

• 2. v1 back-compat. Figure out how to serve the existing v1 getEvents API so current clients keep working unchanged, and implement it. The deprecation timeline is out of scope: it is a product decision.
  fullhistory: index event type and topic count for getEvents v1 #904
  rpcv2: serve v1 getEvents natively on the pager #960

Post-MVP work from this issue has been moved to #995

Depends on

• Revisit getEvents API #426's product direction being settled.
• Method naming convention for the v2 endpoints, tracked in RPC v2 — Roadmap #777 (getEventsV2 as the working name).