GH-9123: SFTP: Use canonicalPath for read operation

Fixes: #9123

The `/` at the beginning of the remote dir path is not necessary when listing files, although it is necessary to download them
The `SftpTemplate.get()` should work also with `remote-dir/MyFile.csv` as input.

* Fix `SftpSession.readRaw()` to call `sftpClient.canonicalPath(source)` if the path does not start with a `/`.
Something similar what is does
* Delegate to `SftpSession.readRaw()` from the `SftpSession.read()`
* Reuse `normalizePath()` for `doList()`

**Auto-cherry-pick to `6.2.x` & `6.1.x`**

spring-integration-sftp/src/test/java/org/springframework/integration/sftp/inbound/SftpInboundRemoteFileSystemSynchronizerTests.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2022 the original author or authors.
+ * Copyright 2002-2024 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -193,8 +193,11 @@ public class SftpInboundRemoteFileSystemSynchronizerTests {
 
 				String[] files = new File("remote-test-dir").list();
 				for (String fileName : files) {
-					when(sftpClient.read("remote-test-dir/" + fileName))
-							.thenReturn(new FileInputStream("remote-test-dir/" + fileName));
+					String remoteFilePath = "remote-test-dir/" + fileName;
+					when(sftpClient.canonicalPath(remoteFilePath))
+							.thenReturn("/" + remoteFilePath);
+					when(sftpClient.read("/" + remoteFilePath))
+							.thenReturn(new FileInputStream(remoteFilePath));
 				}
 				when(sftpClient.readDir("/remote-test-dir")).thenReturn(this.sftpEntries);