Add test runner library

[aakash070]

This change adds a test runner library under tools/celtest which provides native language support for setting up and executing tests.

1. The library makes use of the compiler tool for setting up the CEL runtime environment.
2. It also provides callback options for augmenting the test runner with test suite path, test suite parser, file descriptor set path.

The change also includes a test harness in YAML which aligns with cel.expr.conformance.test.TestSuite.
The following changes will be raised separately:

1. The bazel rule to trigger the test runner with parameterized options
2. Sample usages for triggering test runner library for policies, expressions and compiled expressions.

[TristonianJones]

Please add test cases for the test_runner.go and compiler.go as part of this PR. Thank you!

[aakash070]

@TristonianJones I have added tests for test_runner.go. Tests existed for compiler.go.
I have moved one of the tests for custom policy metadata options from compiler to test_runner. Please check.