decoder: Enable validators to deal with unknown schema

[radeksimko]

Depends on #299

Closes #291

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Comments should be self-explanatory and the implementation works exactly as described/desired in #291 - i.e.

If merging is successful we can continue as normal
If not, we have to skip some validations, like required attribute/block detection
If we encounter a attribute/block that still exists in the schema (must be part of the core schema then), we can return to normal validation for that one

This is all driven by a flag which we provide through the context.