Add fluid container item converter

[repo-alt]

(Volumetric flask, certus tank, universal cell, etc.)
GTNewHorizons/GT-New-Horizons-Modpack#8807

"fluid_hasTag" key may look dubious, but it corresponds to the ConverterItemStack

[asiekierka]

Would it be a better idea to consider appending a fluid map instead, to both make the relationship clearer (item contains a fluid) and to resolve the naming conundrum? Like how enchantments are a list of maps...