Add oauth2 client

lib/build.gradle

@@ -0,0 +1,26 @@
+plugins {
+    id "java-library"
+}
+
+version = "0.0.4"
+archivesBaseName = "tribufu"
+
+repositories {
+    mavenCentral()
+}
+
+dependencies {
+    api "org.apache.commons:commons-math3:3.6.1"
+    implementation "com.google.guava:guava:31.1-jre"
+    testImplementation "org.junit.jupiter:junit-jupiter:5.9.1"
+}
+
+java {
+    toolchain {
+        languageVersion = JavaLanguageVersion.of(11)
+    }
+}
+
+tasks.named("test") {
+    useJUnitPlatform()
+}

lib/src/main/java/com/tribufu/sdk/TribufuClient.java

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+// Copyright (c) Tribufu. All Rights Reserved.
+
+package com.tribufu.sdk;
+
+import java.net.http.HttpClient;
+import java.util.HashMap;
+import java.util.Map;
+
+public class TribufuClient {
+    public static final String VERSION = "0.0.0";
+
+    private final long id;
+    private final String secret;
+    private final HttpClient http;
+    private Map<String, String> defaultHeaders;
+
+    public TribufuClient(long id, String secret) {
+        this.id = id;
+        this.secret = secret;
+
+        String targetTriple = "Java";
+        String userAgent = String.format("Tribufu/%s (+https://api.tribufu.com; %s)", VERSION, targetTriple);
+
+        Map<String, String> headers = new HashMap<>();
+        headers.put("User-Agent", userAgent);
+        headers.put("X-Tribufu-Language", "java");
+        headers.put("X-Tribufu-Version", VERSION);
+
+        this.defaultHeaders = headers;
+
+        this.http = HttpClient.newBuilder()
+                .version(HttpClient.Version.HTTP_2)
+                .build();
+    }
+
+    public long getId() {
+        return this.id;
+    }
+}

lib/src/test/java/com/tribufu/sdk/TribufuClientTest.java

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+// Copyright (c) TribuFu. All Rights Reserved
+
+package com.tribufu.sdk;
+
+import org.junit.jupiter.api.Test;
+import static org.junit.jupiter.api.Assertions.*;
+
+class TribufuClientTest {
+    @Test
+    void clientGetId() {
+        TribufuClient client = new TribufuClient(0, "client_secret");
+        assertEquals(0, client.getId());
+    }
+}