Support for serializing cel.Env to the YAML-serializable env.Config object.

This update introduces mutable setters on the env.Config object, a new method
for marking a library disabled, and a host of YAML serialization tests.

The next piece of this development will be to formalize the extension handlers
as interfaces in the cel package to support the option for configuring a new
CEL environment from the serializable config value and configuring new env options
as appropriate when portions of the config refer to extensions not visible from
the cel package.