Add getFirstEntityByName for FilamentInstance as well

[hannojg]

Currently on an FilamentAsset we have the functionality getFirstEntityByName, which is a very handy helper.
However, this method doesn't seem to exist for FilamentInstance. As we can see here, we only fill a name map for the asset:

filament/libs/gltfio/src/AssetLoader.cpp

Lines 571 to 584 in 3728f06

	// Update the asset's entity list and private node mapping.
	fAsset->mEntities.push_back(entity);
	instance->mEntities.push_back(entity);
	instance->mNodeMap[node - srcAsset->nodes] = entity;
	const char* name = getNodeName(node, mDefaultNodeName);
	if (name) {
	fAsset->mNameToEntity[name].push_back(entity);
	if (mNameManager) {
	mNameManager->addComponent(entity);
	mNameManager->setName(mNameManager->getInstance(entity), name);
	}
	}

I opened this discussion to ask about how this could be implemented for FilamentInstance (I can make the PR, just wanted to first align with you).
Do you think it makes sense for each instance to also have a name map, which we fill when loading the asset? Or is there a better way where we can spare the memory?

Looking forward to working on this! 🙌

[hannojg]

It could probably be also implemented for a FilamentInstance by using the existing node map and the name manager? Would lookup by name be slower for FilamentInstances then?