Document Basic auth header caching, guard against stale copies (#2286)

Motivation:

#2267 introduced `Realm.getBasicAuthHeader()` to cache the computed HTTP
Basic authorization header on immutable `Realm` instances. Two
follow-ups remained: its Javadoc did not clarify that it always
generates a Basic header regardless of the configured `AuthScheme`, and
there was no test verifying that the cached header is scoped to each
`Realm` instance rather than shared across instances.

Modification:

Update the `Realm.getBasicAuthHeader()` Javadoc to explicitly document
that it always encodes credentials as a Basic authorization header,
regardless of the configured scheme. Add a test verifying that two
distinct `Realm` instances with identical credentials produce equal but
distinct cached header instances.

Result:

The behavior of `Realm.getBasicAuthHeader()` is now explicitly
documented, and its per-instance caching is protected by a regression
test against accidental cache sharing.

Author: hyperxpro

client/src/main/java/org/asynchttpclient/Realm.java

@@ -249,7 +249,8 @@ public int getMaxIterationCount() {
 
     /**
      * Returns the lazily computed and cached HTTP Basic authorization header for this immutable realm.
-     * Concurrent first calls may compute the same value more than once.
+     * Always encodes {@link #getPrincipal()} and {@link #getPassword()} as a Basic header, regardless
+     * of {@link #getScheme()}. Concurrent first calls may compute the same value more than once.
      *
      * @return the Basic authorization header
      * @since 3.0.12

client/src/test/java/org/asynchttpclient/util/AuthenticatorUtilsTest.java

@@ -39,6 +39,7 @@
 import static org.junit.jupiter.api.Assertions.assertEquals;
 import static org.junit.jupiter.api.Assertions.assertFalse;
 import static org.junit.jupiter.api.Assertions.assertNotNull;
+import static org.junit.jupiter.api.Assertions.assertNotSame;
 import static org.junit.jupiter.api.Assertions.assertNull;
 import static org.junit.jupiter.api.Assertions.assertSame;
 import static org.junit.jupiter.api.Assertions.assertThrows;
@@ -66,6 +67,19 @@ void preemptiveBasicAuthorizationHeaderIsMemoizedOnRealm() {
         assertSame(first, proxy);
     }
 
+    @Test
+    void basicAuthHeaderCacheIsNotSharedAcrossRealms() {
+        Realm realmA = new Realm.Builder("user", "pass")
+                .setScheme(Realm.AuthScheme.BASIC)
+                .build();
+        Realm realmB = new Realm.Builder("user", "pass")
+                .setScheme(Realm.AuthScheme.BASIC)
+                .build();
+
+        assertEquals(realmA.getBasicAuthHeader(), realmB.getBasicAuthHeader());
+        assertNotSame(realmA.getBasicAuthHeader(), realmB.getBasicAuthHeader());
+    }
+
     @Test
     void computeBodyHashEmptyBody() throws Exception {
         Request request = new RequestBuilder("GET")