Implement SpdxObject Protocol type

[bact]

Implement SpdxObject as a Protocol structural type that will allow creation of SPDX version-agnostic signature of functions.

Per-version generated bindings mean v3_0_1.SHACLObject and v3_1.SHACLObject are distinct nominal types.

To resolve #45

(Note: the generated Go binding from shacl2code use structural type natively (using interface), so it doesn't have this cross-version typing issue. Python Protocol is similar to Go interface; alternatively, we can propose shacl2code to adopt Go pattern for Python, let Python binding use Protocol directly)

[bact]

Looks like it is probably make more sense to implement this upstream in shacl2code. The issue is not SPDX-specific. We can learn from at Go binding generation template.

[bact]

Close this PR.

Revising the implementation into two parts:

1. per-version protocol generations, one protocol per class - it is easier to do this in shacl2code as we can access model information directly from there (will proposed in shacl2code)
2. version-agnostic protocol that merge all versions together - makes more sense to do it here (spdx-python-model) as, at the generation time, shacl2code has no idea about other versions