Refactor mjVFS and resource management.

Resource operations (e.g. mju_openResource, mju_readResource, and
mju_closeResource, etc.) are now all handled by a VFS instance. It
is now up to the VFS to determine which provider to use in order to
handle those operations.

This allows us to dynamically add/remove (aka "mount") providers to
a VFS to handle special requests. mj_addFileVFS and mj_addBufferVFS
have been reimplemented as two such use-cases. Moreover, we expose
the mounting behaviour with two new functions: mj_mountVFS and
mj_unmountVFS.

PiperOrigin-RevId: 856166919
Change-Id: Id723d612ffc0bff020705cf19ef95fbf24676840

test/user/user_resource_test.cc

@@ -301,7 +301,7 @@ TEST_F(ResourceTest, GeneralFailureTest) {
                                           error.data(), error.size());
   ASSERT_THAT(resource, IsNull());
 
-  EXPECT_THAT(error.data(), HasSubstr("could not open"));
+  EXPECT_THAT(error.data(), HasSubstr("Error opening file"));
 }
 
 TEST_F(ResourceTest, NameWithValidPrefix) {