feat: chain with independent convert functions

[kzantow]

Implementing 2-way conversion was not possible in most cases due to Golang package cycle restrictions. This PR moves conversion to functions such that these are able to be defined outside of the packages where the target structs are defined. Take the following example:

models/v1/model.go
models/v2/model.go

.. v1/models cannot refer to v2/models AND have v2/models refer back to v1/models. This PR allows the custom conversion functions to be defined outside the origin structs, for example:

models/v1/model.go
models/v1to2/convert.go
models/v2/model.go

where the functions defined in v1to2/convert are able to reference BOTH v1/model and v2/model, e.g.:

func ConvertV1toV2(v1 v1.Model, v2 *v2.Model) error {
  ....
}

this allows the backwards migrations to also be defined, either in the same or another package, e.g.:

func ConvertV2toV1(v2 v2.Model, v1 *v1.Model) error {
  ....
}