Update extensibility docs for connectors

Author: bart-vmware

docs/docs/v4/connectors/extensibility.md

@@ -1,6 +1,89 @@
 # Extensibility
 
-Connectors were redesigned in Steeltoe v4, and extensibility is not yet available.
-We intend to address this gap in the near future. Please follow along and add any input you may have on [this issue](https://github.com/SteeltoeOSS/Steeltoe/issues/1154).
+Steeltoe Connectors cover a fixed set of supported data stores and messaging systems (PostgreSQL, MySQL, SQL Server, MongoDB, Cosmos DB, RabbitMQ, and Redis/Valkey).
+They are not open-ended plug-ins; extensibility means shaping platform credentials into the connection strings external drivers for those built-in connectors already understand.
+
+Connectors map Cloud Foundry credentials into configuration keys starting with `steeltoe:service-bindings` and merge them with local settings from `Steeltoe:Client`.
+Each connector runs the binding logic for its own service type.
+
+To use a third-party `VCAP_SERVICES` structure, populate the `steeltoe:service-bindings` keys yourself.
+It is recommended to turn off the built-in binding logic to prevent conflicts by setting `SkipDefaultServiceBindings` to `true`.
+Doing so will still merge with local settings from `Steeltoe:Client`.
+
+> [!TIP]
+> See the [Cloud Foundry configuration provider](../configuration/cloud-foundry-provider.md) for `vcap:*` keys and [`VCAP_SERVICES`](https://docs.cloudfoundry.org/devguide/deploy-apps/environment-variable.html#VCAP-SERVICES) in general.
+
+For example, to use a third-party Cloud Foundry service broker for PostgreSQL that sets the `VCAP_SERVICES` environment variable to:
+
+```json
+{
+  "custom-postgres-broker": [
+    {
+      "name": "products-db",
+      "credentials": {
+        "custom-hostname-key": "example.cloud.com",
+        "custom-port-key": 2345,
+        "custom-username-key": "products-user",
+        "custom-password-key": "products-secret",
+        "custom-database-name-key": "product-database"
+      }
+    },
+    {
+      "name": "orders-db",
+      "credentials": {
+        "custom-hostname-key": "example.cloud.com",
+        "custom-port-key": 2345,
+        "custom-username-key": "orders-user",
+        "custom-password-key": "orders-secret",
+        "custom-database-name-key": "order-database"
+      }
+    }
+  ]
+}
+```
+
+The following code can be used to map the PostgreSQL credentials to the format that [`NpgsqlConnectionStringBuilder`](https://www.npgsql.org/doc/api/Npgsql.NpgsqlConnectionStringBuilder.html) expects:
+
+```c#
+using Npgsql;
+using Steeltoe.Configuration.CloudFoundry;
+using Steeltoe.Connectors;
+using Steeltoe.Connectors.PostgreSql;
+
+var builder = WebApplication.CreateBuilder();
+builder.AddCloudFoundryConfiguration();
+MapCustomServiceBindings("custom-postgres-broker");
+builder.AddPostgreSql(configure => configure.SkipDefaultServiceBindings = true, null);
+var app = builder.Build();
+
+var factory = app.Services.GetRequiredService<ConnectorFactory<PostgreSqlOptions, NpgsqlConnection>>();
+
+PostgreSqlOptions productsDbOptions = factory.Get("products-db").Options;
+Console.WriteLine(productsDbOptions.ConnectionString);
+// Database=product-database;Host=example.cloud.com;Password=products-secret;Port=2345;Username=products-user
+
+PostgreSqlOptions ordersDbOptions = factory.Get("orders-db").Options;
+Console.WriteLine(ordersDbOptions.ConnectionString);
+// Database=order-database;Host=example.cloud.com;Password=orders-secret;Port=2345;Username=orders-user
+
+void MapCustomServiceBindings(string brokerName)
+{
+    var options = builder.Configuration.GetSection("vcap").Get<CloudFoundryServicesOptions>();
+
+    foreach (CloudFoundryService service in options?.Services
+        .Where(pair => pair.Key == brokerName)
+        .SelectMany(pair => pair.Value) ?? [])
+    {
+        builder.Configuration.AddInMemoryCollection(new Dictionary<string, string?>
+        {
+            // Map credentials into the property names expected by NpgsqlConnectionStringBuilder.
+            [$"steeltoe:service-bindings:postgresql:{service.Name}:host"] = service.Credentials["custom-hostname-key"].Value,
+            [$"steeltoe:service-bindings:postgresql:{service.Name}:port"] = service.Credentials["custom-port-key"].Value,
+            [$"steeltoe:service-bindings:postgresql:{service.Name}:username"] = service.Credentials["custom-username-key"].Value,
+            [$"steeltoe:service-bindings:postgresql:{service.Name}:password"] = service.Credentials["custom-password-key"].Value,
+            [$"steeltoe:service-bindings:postgresql:{service.Name}:database"] = service.Credentials["custom-database-name-key"].Value
+        });
+    }
+}
+```
 
-See [Advanced settings](usage.md#advanced-settings) to customize the built-in Connectors.