Protobuf schema references support

[jjaakola-aiven]

About this change - What it does

Add support for protobuf schema references

References: #195

Why this way

Initial work is in pull request #467
Original #467 pull request description:

These changes add basic support of references/dependencies for protobuf schema type.
This code contains much common code which allows adding support of references/dependencies to other types.
Limitations:
The compatibility check does not support references/dependencies.
Serialization/Deserialization of protobuf does not support references/dependencies.

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[libretto]

@jjaakola-aiven do You think it is a good idea to develop two different ways of the same functionality? I just have already a workaround for compatibility(compare) functionality in the Instaclustr repository and planning to release it ASAP.

[libretto]

We have a type for dependencies... and my code which I working on already use there exactly this type. please not that Your fixes will interfere with my code logic which I just debugging. You can see it at instaclustr#24

[jjaakola-aiven]

@libretto, this PR does not implement the same functionality in different way. Your PR and commit is the basis and my commits are on top of it. I understand this creates a conflict to your implementation, but I have to consider the total quality of the feature and therefore decided to add tests and relevant changes.

[libretto]

@jjaakola-aiven Better add your tests to my latest code where most bugs (including reported by You) are fixed.

[libretto]

@libretto, this PR does not implement the same functionality in different way. Your PR and commit is the basis and my commits are on top of it. I understand this creates a conflict to your implementation, but I have to consider the total quality of the feature and therefore decided to add tests and relevant changes.

@jjaakola-aiven Actually, it did. You did the same work which I did too You can see the code in my PR. It adds more work for me for resolving conflicts and rewriting code which I did in time You did Your changes. You did something in a different way. For example, I have 2 classes one for References and one for Dependencies. I understand You like use List[Reference] instead of References class. It is ok and I will replace it (actually it is not a bad style because karapace has another example of using similar array classes, but ok). By logic, it is better to have Dependency in a separate class because dependencies are resolved references. I understand You want to improve the code style. But why You can not simply tell me to do that by pointing out what I must change? I understand You want to add more tests but why You cannot add them in separate files to avoid conflicts with my code?