Make many-to-many entity saving more idiomatic

[ra0x3]

• We previously added support for many-to-many (M2M) relationships https://github.com/FuelLabs/fuel-indexer/issues/947
• As a quick win, we just implemented this via string hacking (here)
• Now that this functionality is stable and works, we should make it a bit more idiomatic
• We originally used string hacking over FtColumn because we didn't (and still don't) need a FtColumn::Object of serialized FtColumns, because we're merely saving references (i.e., IDs) of items, and not the items themselves
• However, we should replace the string hacking with the use of FtColumn
  • Foreign keys should use FtColumn::ForeignKey