Support for Custom Conformance Environments

[TristonianJones]

Currently, the conformance tests operate on the CEL standard environment. With the implementation of google/cel-go#316, it has become clear that extensions to core CEL behavior would benefit from being able to use the conformance framework as well.

In order to support his case and clearly identify what environment the conformance test is operating on, it would be ideal to be able to specify the name fo the environment, perhaps as it is understood by the conformance implementer, against which the expressions should be compiled and evaluated.

The feature would enable applications with custom CEL environments to test their extensions in a uniform way across implementations (e.g. Go, C++)