GraphQL changes to enable pruning (#20523)

## Description 

- main changes from #19237 and
this is the only unknown blocker
- plus rebase, addressing comments and some testing changes.

## Test plan 

cargo nextest run --no-capture sui-graphql-e2e-tests::tests
run_test::stable/prune

---

## Release notes

Check each box that your changes affect. If none of the boxes relate to
your changes, release notes aren't required.

For each box you select, include information after the relevant heading
that describes the impact of your changes that a user might notice and
any actions they must take to implement updates.

- [ ] Protocol: 
- [ ] Nodes (Validators and Full nodes): 
- [ ] Indexer: 
- [ ] JSON-RPC: 
- [X] GraphQL: graphql changes to enable pruning, mainly around
watermark
- [ ] CLI: 
- [ ] Rust SDK:
- [ ] REST API:

crates/sui-graphql-e2e-tests/tests/stable/prune/prune.exp

@@ -48,10 +48,13 @@ task 10, line 43:
 //# advance-epoch
 Epoch advanced: 2
 
-task 11, lines 45-55:
+task 11, lines 45-58:
 //# run-graphql --wait-for-checkpoint-pruned 4
 Response: {
   "data": {
+    "epoch": {
+      "epochId": 2
+    },
     "checkpoints": {
       "nodes": [
         {
@@ -71,7 +74,7 @@ Response: {
   }
 }
 
-task 12, lines 57-67:
+task 12, lines 60-70:
 //# run-graphql
 Response: {
   "data": {
@@ -86,15 +89,15 @@ Response: {
   }
 }
 
-task 13, lines 69-72:
+task 13, lines 72-75:
 //# run-graphql
 Response: {
   "data": {
     "chainIdentifier": "084d40f2"
   }
 }
 
-task 14, lines 74-103:
+task 14, lines 77-106:
 //# run-graphql
 Response: {
   "data": {

crates/sui-graphql-e2e-tests/tests/stable/prune/prune.move

@@ -44,6 +44,9 @@ module Test::M1 {
 
 //# run-graphql --wait-for-checkpoint-pruned 4
 {
+  epoch {
+    epochId
+  }
   checkpoints {
     nodes {
       epoch {