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/src/index.ts

@@ -3,6 +3,7 @@ export * from './blockhash';
 export * from './bpf-loader-deprecated';
 export * from './bpf-loader';
 export * from './connection';
+export * from './epoch-schedule';
 export * from './fee-calculator';
 export * from './keypair';
 export * from './loader';