feat: add convenience methods to EpochSchedule (#17810)

* first try, failing test

* fix implementation and tests

* lint:fix

* move method tests to seperate test

* lint fix

* apply starry's comments and grab the bonus points

* minor fixes after starry's second review

Co-authored-by: Arrowana <8245419+Arrowana@users.noreply.github.com>

web3.js/test/connection.test.ts

@@ -9,6 +9,7 @@ import {
   Account,
   Authorized,
   Connection,
+  EpochSchedule,
   SystemProgram,
   Transaction,
   LAMPORTS_PER_SOL,
@@ -24,7 +25,6 @@ import {
   BLOCKHASH_CACHE_TIMEOUT_MS,
   Commitment,
   EpochInfo,
-  EpochSchedule,
   InflationGovernor,
   SlotInfo,
 } from '../src/connection';