(May be easier to review by commit.)

@rokonec @JanKrivanek I ended up reworking the PR, please take another look when you get a chance.

Here is the key revelation that led to the current approach:

IConvertible provides methods to convert the implementing type to primitives, most importantly to string. It does not include the other direction (I had assumed it was a convention with a string-taking constructor, for example), i.e. Convert.ChangeType(o, typeof(T)) fails for custom types. It works only for "built-in" convertibles, of which there is a limited set. They are enumerated in the System.TypeCode enum.

As a consequence, custom IConvertible implementations are allowed to be used only as task outputs, as the engine can convert from the output value to the internal string representation. The set of allowed task inputs types is therefore limited to built-in convertibles and there is no need to communicate the parameter type as part of the payload or infer it from the signature of the corresponding .NET property.

TaskParameter now distinguishes:

1. PrimitiveType and PrimitiveTypeArray types, which are converted with Convert.ChangeType(o, typeof(T)) to/from string or serialized natively if the type is supported by ITranslator.
2. ValueType and ValueTypeArray types, which are converted only to string because they can only be task outputs.
3. ITaskItem, ITaskItemArray, Invalid, Null - same as before.

PrimitiveType and PrimitiveTypeArray additionally serializes the TypeCode of the specific type. I did this to avoid exploding TaskParameterType enum with many more types.

Since this functionality is somewhat obscure, I did my best to cover it with tests, doing some refactoring along the way. For example, TaskParameter_Tests could be easily parameterized using [Theory] instead of many [Fact]s with duplicated code.

The ValueType and ValueTypeArray conversion code is still under a change wave check. At this point I am more confident that the new logic is correct so I'll defer to your opinion whether it's needed or not.

(May be easier to review by commit.)

No longer the case ☹️