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: 861550939
Change-Id: I070eb4bcc2982466c8f368f7918005538baa5185

test/xml/xml_api_test.cc

@@ -126,7 +126,7 @@ TEST_F(LoadXmlTest, InvalidFileFails) {
   std::array<char, 1000> error;
   mjSpec* spec = mj_parseXML("invalid", nullptr, error.data(), error.size());
   EXPECT_THAT(spec, IsNull()) << "Expected model loading to fail.";
-  EXPECT_THAT(error.data(), HasSubstr("No such file or directory"));
+  EXPECT_THAT(error.data(), HasSubstr("Error opening file"));
 }
 
 TEST_F(MujocoTest, SaveXmlShortString) {