Allow for custom serialization layers

[sirthias]

We are using LMDB as the primary persistence layer for a number of small to medium sized applications and are thus looking into zio-ldmb. In principle it fits our bill quite well with one big exception: We are serializing all our data to CBOR rather than JSON (using borer).

It seems to us that a layer like zio-lmdb should be agnostic to the serialization logic rather than hard-wired to JSON. How hard would it be to introduce another type-class layer (for example) abstracting over the (de)serialization logic? zio-json could still be the default, if need be, but it'd be great if we could plug our own custom or alternative implementation.