Add /installed_extensions endpoint to collect statistics about extension usage. (#8917)

Add /installed_extensions endpoint to collect
statistics about extension usage.
It returns a list of installed extensions in the format:

```json
{
  "extensions": [
    {
      "extname": "extension_name",
      "versions": ["1.0", "1.1"],
      "n_databases": 5,
    }
  ]
}
```

---------

Co-authored-by: Heikki Linnakangas <heikki@neon.tech>

libs/compute_api/src/responses.rs

@@ -1,5 +1,6 @@
 //! Structs representing the JSON formats used in the compute_ctl's HTTP API.
 
+use std::collections::HashSet;
 use std::fmt::Display;
 
 use chrono::{DateTime, Utc};
@@ -155,3 +156,15 @@ pub enum ControlPlaneComputeStatus {
     // should be able to start with provided spec.
     Attached,
 }
+
+#[derive(Clone, Debug, Default, Serialize)]
+pub struct InstalledExtension {
+    pub extname: String,
+    pub versions: HashSet<String>,
+    pub n_databases: u32, // Number of databases using this extension
+}
+
+#[derive(Clone, Debug, Default, Serialize)]
+pub struct InstalledExtensions {
+    pub extensions: Vec<InstalledExtension>,
+}