Propagate backlinks on Signal and Signal-with-Start responses (#9897)

## What changed?

### **High level**

With https://github.com/temporalio/api/pull/761 to add the linking on
the Signal and Signal-with-Start responses, This PR adds logic from the
server that:
* Adds `requestID` from Signal and Signal-with-Start requests to the
CHASM workflow tree under a new map field `IncomingSignals`, and event
store, so these requestIDs stay in buffer
* Return a backlink in the response that references the `requestID`
* On buffer flush to the DB transaction, attach these `requestID` to a
concrete `eventID`, which would allow users to later know which event
correlated w/ this request. We will wire the concrete event ID to the
signal request IDs stored in the workflow component CHASM tree
(`IncomingSignals` map)

> [!NOTE]
> Feature is gated behind a new dynamicconfig
`EnableCHASMSignalBacklinks`, which implicitly is only checked if
`EnableChasm` is enabled.

## Why?

This will enable the caller of the signal to have a backlink to the
cross-namespace signal invoked, which will become more relevant for
Nexus SDK ergonomics.

## How did you test it?
- [ ] built
- [ ] run locally and tested manually
- [ ] covered by existing tests
- [ ] added new unit test(s)
- [x] added new functional test(s)

In functional tests, I augmented existing tests for Signal and
Signal-with-Start to:
* Ensure that backlink is returned via the responses
* Later use `DescribeWorkflow` to ensure that we get a concrete EventID
(mapped when buffer flushed)
* Multiple signals with the same `requestID` gets de-duped

```
$ go test ./tests/ -run TestLinksTestSuite
ok      go.temporal.io/server/tests     1.486s
```

```
$ go test ./tests/ -run 'TestNexusWorkflowTestSuite' -count=1
ok      go.temporal.io/server/tests     4.714s
```

## Potential risks

Need to test end-to-end to see that the link shows up correctly in the
Web UI.

Feature is gated behind dynamicconfig since it requires CHASM-based
workflow to be enabled.

service/history/configs/config.go

@@ -72,6 +72,7 @@ type Config struct {
 	MaxCallbacksPerExecution              dynamicconfig.IntPropertyFnWithNamespaceFilter
 	EnableChasm                           dynamicconfig.BoolPropertyFnWithNamespaceFilter
 	EnableCHASMCallbacks                  dynamicconfig.BoolPropertyFnWithNamespaceFilter
+	EnableCHASMSignalBacklinks            dynamicconfig.BoolPropertyFnWithNamespaceFilter
 	ChasmMaxInMemoryPureTasks             dynamicconfig.IntPropertyFn
 	EnableCHASMSchedulerCreation          dynamicconfig.BoolPropertyFnWithNamespaceFilter
 	EnableCHASMSchedulerMigration         dynamicconfig.BoolPropertyFnWithNamespaceFilter
@@ -500,8 +501,9 @@ func NewConfig(
 		EnableCHASMSchedulerCreation:  dynamicconfig.EnableCHASMSchedulerCreation.Get(dc),
 		EnableCHASMSchedulerMigration: dynamicconfig.EnableCHASMSchedulerMigration.Get(dc),
 
-		EnableCHASMCallbacks:    dynamicconfig.EnableCHASMCallbacks.Get(dc),
-		ExternalPayloadsEnabled: dynamicconfig.ExternalPayloadsEnabled.Get(dc),
+		EnableCHASMCallbacks:       dynamicconfig.EnableCHASMCallbacks.Get(dc),
+		EnableCHASMSignalBacklinks: dynamicconfig.EnableCHASMSignalBacklinks.Get(dc),
+		ExternalPayloadsEnabled:    dynamicconfig.ExternalPayloadsEnabled.Get(dc),
 
 		EventsShardLevelCacheMaxSizeBytes: dynamicconfig.EventsCacheMaxSizeBytes.Get(dc),          // 512KB
 		EventsHostLevelCacheMaxSizeBytes:  dynamicconfig.EventsHostLevelCacheMaxSizeBytes.Get(dc), // 256MB