Implementing opaque fields

[giograno]

In our application, we might want to "disable" Avro schema for some fields.
Unfortunately, we can't completely ignore a field (similar to what transient does in the Java world) but we can simply tell Avro to serialize everything to a string.

We introducing the type Opaque that, used in combination with Annotated, tells py-avro reduce everything to a string.
For this to work end-to-end, we need to implement a similar logic in our ObjectSerializer (see comment below).

💡 : we have being adding features working across this repo and the mapper component in LocalStack. In the future, we might want to either have an external comprehensive library with both components, or internalize everything.

Related to PNX-443

[thrau]

i like this approach! i didn't actually know about typing.Annotated, nice find! i love that there's still things to learn about python :D

we should give this a try, but i'd love to see a concrete example from localstack soon so we can decide whether this is the way forward.

remember that these types of changes will proliferate through the entire codebase