[ENH] Add Ability to Dynamically Specify Input Values for Activities

One thing I find myself spending a decent amount of time on is what's possible for an input and if autocomplete can help.

One use case in particular is interesting - ability to specify the values that populate a dropdown. Today I see `IActivityPropertyDefaultValueProvider` but that sounds life it's just for a default value. A `IActivityPropertyValueProvider` sounds more appropriate? If it was a textbox then it would be autocomplete when a value provider is specified.

**Main value:** Being able to give the user a choice of available options for a textbox or dropdown via an API call or database query, etc. gives the user the possibility to know what should be entered in the dropdown.

```c#
[Input(
DisplayName = "Select an option",
UIHint = InputUIHints.DropDown,
ValueProvider = typeof(SomeValueProvider)
)]
public Input Region { get; set; };
```