Expose interpreter.Activation creation methods on 'cel' package.

[TristonianJones]

Feature request checklist

• There are no issues that match the desired change
• The change is large enough it can't be addressed with a simple Pull Request
• If this is a bug, please file a Bug Report.

Change
Expose the interpreter.Activation via the top-level cel package, either via the creation methods interpreter.NewActivation as cel.NewActivation or via type alias.

Example
Replace this text with an example indicating the desired functionality.