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/epoch-schedule.test.ts

@@ -0,0 +1,46 @@
+import {expect} from 'chai';
+
+import {EpochSchedule} from '../src';
+
+describe('EpochSchedule', () => {
+  it('slot methods work', () => {
+    const firstNormalEpoch = 14;
+    const firstNormalSlot = 524_256;
+    const leaderScheduleSlotOffset = 432_000;
+    const slotsPerEpoch = 432_000;
+    const warmup = true;
+
+    const epochSchedule = new EpochSchedule(
+      slotsPerEpoch,
+      leaderScheduleSlotOffset,
+      warmup,
+      firstNormalEpoch,
+      firstNormalSlot,
+    );
+
+    expect(epochSchedule.getEpoch(35)).to.be.equal(1);
+    expect(epochSchedule.getEpochAndSlotIndex(35)).to.be.eql([1, 3]);
+
+    expect(
+      epochSchedule.getEpoch(firstNormalSlot + 3 * slotsPerEpoch + 12345),
+    ).to.be.equal(17);
+    expect(
+      epochSchedule.getEpochAndSlotIndex(
+        firstNormalSlot + 3 * slotsPerEpoch + 12345,
+      ),
+    ).to.be.eql([17, 12345]);
+
+    expect(epochSchedule.getSlotsInEpoch(4)).to.be.equal(512);
+    expect(epochSchedule.getSlotsInEpoch(100)).to.be.equal(slotsPerEpoch);
+
+    expect(epochSchedule.getFirstSlotInEpoch(2)).to.be.equal(96);
+    expect(epochSchedule.getLastSlotInEpoch(2)).to.be.equal(223);
+
+    expect(epochSchedule.getFirstSlotInEpoch(16)).to.be.equal(
+      firstNormalSlot + 2 * slotsPerEpoch,
+    );
+    expect(epochSchedule.getLastSlotInEpoch(16)).to.be.equal(
+      firstNormalSlot + 3 * slotsPerEpoch - 1,
+    );
+  });
+});