What is the ParameterResolver extension interface and how do you use it for custom injection?

// Custom ParameterResolver: inject a configured HttpClient
public class HttpClientExtension
        implements ParameterResolver, BeforeEachCallback, AfterEachCallback {

    private final Map<ExtensionContext, HttpClient> clients = new ConcurrentHashMap<>();

    @Override
    public boolean supportsParameter(
            ParameterContext paramCtx,
            ExtensionContext extCtx) {
        // Only resolve HttpClient parameters
        return paramCtx.getParameter().getType() == HttpClient.class;
    }

    @Override
    public Object resolveParameter(
            ParameterContext paramCtx,
            ExtensionContext extCtx) {
        // Create and return an HttpClient for this test
        HttpClient client = HttpClient.newBuilder()
            .connectTimeout(Duration.ofSeconds(5))
            .build();
        clients.put(extCtx, client);
        return client;
    }

    @Override
    public void afterEach(ExtensionContext ctx) {
        HttpClient client = clients.remove(ctx);
        if (client != null) client.close();
    }
}

// Usage:
@ExtendWith(HttpClientExtension.class)
class ApiIntegrationTest {

    @Test
    void fetchUsers(HttpClient client) {  // injected by extension!
        HttpResponse<String> resp = client.send(
            HttpRequest.newBuilder(URI.create("https://api.example.com/users")).build(),
            HttpResponse.BodyHandlers.ofString()
        );
        assertEquals(200, resp.statusCode());
    }
}