Support for WDL's Object type

[stxue1]

Objects are deprecated in 1.1, but the spec, at least according to their unit tests, requires the object type to be supported for WDL 1.1. There is a unit test in the SPEC:
https://github.com/openwdl/wdl/blob/9c0b9cf4586508a9e6260cc5c5e562e21f625aac/SPEC.md?plain=1#L1054-L1091

And according to the wdl-tests specification, this test is required:
https://github.com/openwdl/wdl-tests/blob/58ff36209586ed69c9a64d3e0b151a343f12a4eb/docs/Specification.md?plain=1#L78-L81

The priority of the test. Must be one of the following values. Defaults to "required".

Does MiniWDL ever plan on supporting objects? Objects already "exist" and appear to only be used for coercing to structs, but the type isn't used during MiniWDL's parsing step, so running any workflow with objects fails before actually running it.

(/home/heaucques/Documents/wdl-conformance-tests/unit_tests/test_object.wdl Ln 5 Col 5) Unknown type Object
        Object obj = object {
        ^^^^^^^^^^^^^^^^^^^^^

[mlin]

@stxue1 Indeed we have documented this as a limitation. Behind the scenes, the dynamically typed nature of Object would necessitate pervasive special-casing in the static type-checking logic and we judged this would not be a good trade-off given the introduction of structs to replace them.