Implement Data Type Extensions

[setrofim]

CoRIM spec defines two types of extensibility:

• Map Extensions, indicated by $$NAME-extension, allow extending map structures defined in the spec by adding new data fields. This is already covered by ExtensionMap.
• Data Type Extensions, indicated by $NAME-type-choice, allow extending structures defined in the spec with additional data types for existing fields. This is currently not supported.

(Note: there is also a third kind, due to CoSWID -- "enum extensions", indicated by $NAME (without the -type-choice suffix), which allows specifying addition values to defined enumerations, such as $role. This, too, is already covered).

To support Data Type Extensions, we will need to

• Add Extension(ExtensionValue<'a>) variant to the type-choice enums.
• On deserialisation, whenever we don't recognise the data, populate this variant, instead of raising an error.
• Add a TaggedValue(u64, ExtensionsValue<'a>) variant to ExtensionValue to support new tagged types (this would be the norm for type-choice extensions).

[setrofim]

#29 adds tags support for ExtensionValues, and also implements a PoC extension support for MeasuredElementTypeChoice and ClassIdTypeChoice.

This is the full list of types extensions support needs to be added to:

• concise-rim-type-choice
• concise-tag-type-choice
• corim-role-type-choice
• corim-id-type-choice
• profile-type-choice
• entity-name-type-choice
• crypto-key-type-choice
• measured-element-type-choice
• class-id-type-choice
• comid-role-type-choice
• domain-type-choice
• instance-id-type-choice
• group-id-type-choice
• raw-value-type-choice
• tag-id-type-choice
• tag-rel-type-choice