Adds a new 'extensions' sub-section to the 'metrics' section of serverStatus

Part of several diagnostics added for extensions - the 'observability' tab of this program document[1] include the full details.

[1]: https://docs.google.com/document/d/1Lu6Op7yQBWQQxhnhKBsEyl_WR0UHWgmrN-u0nijJGfk/edit?tab=t.v4x6caquccnc#heading=h.qumf1bd8is7b

Description of Linked Ticket

I found that these metrics were under a top-level section of serverStatus like this:

```
"extension.vectorSearch" :

{ "legacyVectorSearchUsed" : NumberLong(0), "extensionVectorSearchUsed" : NumberLong(0), "onViewKickbackRetries" : NumberLong(0), "inSubpipelineKickbackRetries" : NumberLong(0) }

,
```

I think we intended for them to be nested - and in SERVER-115695 we added this section under the top-level 'metrics':
```
"extension": {
"totalAggStageExecMicros": NumberLong(0);
}
```

I think taken together we should move these metrics and have it look like:
```
"extension": {
"totalAggStageExecMicros": NumberLong(0);
// Metrics coming in SERVER-115696
"extensionSuccesses": NumberLong(0);
"extensionFailures": NumberLong(0);
// These metrics:
"vectorSearch" :

{ "legacyVectorSearchUsed" : NumberLong(0), "extensionVectorSearchUsed" : NumberLong(0), "onViewKickbackRetries" : NumberLong(0), "inSubpipelineKickbackRetries" : NumberLong(0) }

,
```