Dispatcher refactor to improve function declaration, definition, and extension

[TristonianJones]

This change introduces a top-level cel.Type as well as functional options for configuring cel.Variable and cel.Function definitions using go-native APIs.

This change is a shift away from the existing protobuf APIs which is also provides the following improvements:

• Automatic generation of function overload type-guards to provide basic invocation protections
• Automatic generation of dynamic dispatch functions for resolving overloads at runtime (for both parse-only expressions and dynamically typed data)
• Ability to declare and define functions within a single context.

The only drawback of this approach is that there is some minimal invocation overhead for functions declared/defined in this manner. Future optimizations may permit users to disable such type-guards if they deem them not to be necessary.

As a follow up PR, the ext function libraries and CEL standard operators will be shifted over to this new declaration style to improve the ease of extension of the core library implementations.

This is the basis for the feature requested in #493