Support a Metadata's option to load/save unsafe YAML types

[roll]

Overview

See the original issue - #608

The same as we have done with metadata.to_json(...encoder_class) we can give users an ability to store non-primitive types in YAML descriptors. It's not interoperable but can be useful within some projects e.g. working with numpy.

By default, I think we should load only safe types. The logic is similar to Package/Resource(trusted=True) when the user can allow unsafe paths.

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Actually, I guess we can re-use this trusted flag, pass it to Metadata and allow unsafe types loading alongside unsafe paths.