Add IDTokenSource Interface (#433)

## What changes are proposed in this pull request?
This PR adds the foundational components for OIDC support in the
Databricks Java SDK:

**IDToken Class:** Encapsulates ID tokens from identity providers for
OAuth flows.
**IDTokenSource** Interface: Defines a standard for retrieving ID
tokens.

## How is this tested?
- Unit tests added for the IDToken class.
- IDTokenSource is not tested, as no implementation exists yet.

NO_CHANGELOG=true

---------

Co-authored-by: Parth Bansal <parth.bansal@databricks.com>

Author: emmyzhou-db

databricks-sdk-java/src/main/java/com/databricks/sdk/core/oauth/IDToken.java

@@ -0,0 +1,31 @@
+package com.databricks.sdk.core.oauth;
+
+/**
+ * Represents an ID Token provided by an identity provider from an OAuth flow. This token can later
+ * be exchanged for an access token.
+ */
+public class IDToken {
+  // The string value of the ID Token
+  private final String value;
+
+  /**
+   * Constructs an IDToken with a value.
+   *
+   * @param value The ID Token string.
+   */
+  public IDToken(String value) {
+    if (value == null || value.isEmpty()) {
+      throw new IllegalArgumentException("ID Token value cannot be null or empty");
+    }
+    this.value = value;
+  }
+
+  /**
+   * Returns the value of the ID Token.
+   *
+   * @return The string representation of the ID Token.
+   */
+  public String getValue() {
+    return value;
+  }
+}

databricks-sdk-java/src/main/java/com/databricks/sdk/core/oauth/IDTokenSource.java

@@ -0,0 +1,12 @@
+package com.databricks.sdk.core.oauth;
+
+/** IDTokenSource is anything that returns an IDToken given an audience. */
+public interface IDTokenSource {
+  /**
+   * Retrieves an ID Token for the specified audience.
+   *
+   * @param audience The intended recipient of the ID Token.
+   * @return An {@link IDToken} containing the token value.
+   */
+  IDToken getIDToken(String audience);
+}

databricks-sdk-java/src/test/java/com/databricks/sdk/core/oauth/IDTokenTest.java

@@ -0,0 +1,22 @@
+package com.databricks.sdk.core.oauth;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+
+import org.junit.jupiter.api.Test;
+
+public class IDTokenTest {
+
+  private static final String accessToken = "testIdToken";
+
+  @Test
+  void testIDTokenWithValue() {
+    IDToken idToken = new IDToken(accessToken);
+    assertEquals(accessToken, idToken.getValue());
+  }
+
+  @Test
+  void testIDTokenWithNullValue() {
+    assertThrows(IllegalArgumentException.class, () -> new IDToken(null));
+  }
+}