Add a separate field for expressions under input bindings map (#449)

* Add test_suite and test_config under conformance

* added test_config.pb.go and test_suite.pb.go to conforanmce library build target

* added test suite messages to simple.proto and updated test_config to env_config

* add uncommitted files

* updated build files for generated conformance and test code

* corrected field name in env_config

* replaced Variable and Function messages with cel.expr.DEcl

* resolved comments

* removed redundant any from simple proto

* added context_message as an input kind to test case

* removed redundant anypb import in env_config

* update type_name to context_variable_type to conform with linting rules

* moved test suite, test section and test case to suite.proto under conformance/test

* update context variable attribute name to type_name

* removed redundant FileDescriptor field from config

* Revert "removed redundant FileDescriptor field from config"

This reverts commit 3757b44.

* added separate identifier field for expression type inputs to test case

* corrected comment on input value expr

* corrected comment on input value expr

* removed nested messages and added those as independent messages

Author: aakash070