develop a way to specify specific proto messages within the resource model.

[toumorokoshi]

motivation: #66 (comment).

aepc now uses resource representation in aep-lib-go: https://github.com/aep-dev/aep-lib-go/blob/main/examples/resource-definitions/bookstore.yaml#L33.

So now the proto is defined via an extended jsonschema. The challenge is to map these types to the appropriate proto message: that has to be explicit, since a message is within a package while a json-schema is a representation of the object (although of course there's $ref).

So I think we need a new extension to ensure that, when the proto is being generated, the appropriate type is loaded and used. This would allow one to specify the aep.types.Money message.