Support eventstream example (light_client_*_update) from recent beacon API test suite

[jeluard]

Beacon APIs v2.4.0 introduce new eventstream examples #1 and #2 that do not pass currently on lodestar.

  1) eventstream event data
       light_client_finality_update:
     Error: Invalid slot=0 fork=phase0 for lightclient fork types
      at Object.getLightClientForkTypes (file:///Users/julien/Documents/Projects/chainsafe/lodestar/packages/config/lib/forkConfig/index.js:95:23)
      at getLightClientTypeFromHeader (file:///Users/julien/Documents/Projects/chainsafe/lodestar/packages/api/src/beacon/routes/events.ts:71:23)
      at Object.toJson (file:///Users/julien/Documents/Projects/chainsafe/lodestar/packages/api/src/beacon/routes/events.ts:117:31)
      at Object.toJson (file:///Users/julien/Documents/Projects/chainsafe/lodestar/packages/api/src/beacon/routes/events.ts:136:30)
      at Context.<anonymous> (file:///Users/julien/Documents/Projects/chainsafe/lodestar/packages/api/test/unit/beacon/oapiSpec.test.ts:174:47)
      at process.processImmediate (node:internal/timers:478:21)