GH-47560: [C++] Fix host handling for default HDFS URI (#47458)

### Rationale for this change

In #25324 a fix is introduced for the python HadoopFileSystem, but it does not work if you use `from_uri()`, as it is passed to the underlying C++ implementation of the options parsing. The "default" case is not handled as in the python case, as the whole "hdfs://default" is passed to the underlying hdfs library, that expect "default" to search in `$HADOOP_CONF_DIR/core-site.xml`.

### What changes are included in this PR?

Handle the `HadoopFileSystem.from_uri()` (or `FileSystem.from_uri()` when using `hdfs://default:xxx`) special HDFS URIs.

### Are these changes tested?

There are no specific tests for this feature, but existing HDFS CI jobs pass.

### Are there any user-facing changes?

Not exactly, but the documentation is honored for the `from_uri()` case.

* GitHub Issue: #47560

Lead-authored-by: Diego Sevilla Ruiz <[email protected]>
Co-authored-by: Antoine Pitrou <[email protected]>
Signed-off-by: Antoine Pitrou <[email protected]>

Author: dsevilla

cpp/src/arrow/filesystem/hdfs.cc

@@ -363,8 +363,14 @@ Result<HdfsOptions> HdfsOptions::FromUri(const Uri& uri) {
     options_map.emplace(kv.first, kv.second);
   }
 
+  // Special case host = "default" or "hdfs://default" as stated by GH-47560.
+  // If given the string "default", libhdfs selects the default filesystem
+  // from `core-site.xml`.
   std::string host;
-  host = uri.scheme() + "://" + uri.host();
+  if (uri.host() == "default")
+    host = uri.host();
+  else
+    host = uri.scheme() + "://" + uri.host();
 
   // configure endpoint
   const auto port = uri.port();