Function support

[jaredoconnell]

Changes introduced with this PR

This PR adds functions to the schema.

There are two main categories of functions. Statically typed and dynamically typed.

• The way that the dynamically typed functions are implemented mean that we still validate their type, but a little looser using a function that determines the type after the input parameters are passed in.
• Dynamically typed functions allow functions that support more generic things, like creating an object with specific fields.
• The statically typed functions are validated strictly. You pass in the function handler, and it's validated at runtime.
• The allowed return types for a standard function may or may not include an error return. This error return is in addition to the single return specified by the schema, if present.

For examples on how this is used, see the expressions draft PR: arcalot/arcaflow-expressions#16

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[dbutenhof]

Just some comments, mostly on messaging...

[webbnh]

Looks good. I just have a couple of questions and a couple of suggestions (which you can consider or ignore if don't have any other changes to make).