Remove raw_string output type

What does this MR do?

This MR removes the raw_string output type, as discussed at #128 (comment 2128759088).

Tests have been changed to convert raw_string outputs to string, the default output is now string. Steps are now required to write all values as JSON.

The end-to-end test has been updated to use a version of echo-step that does not use raw_string. The integration tests in runner_integration_test.go have been removed as they are duplicates of the e2e test, and those in cache_integration_test.go have been replaced with unit tests in git_test.go and cache_test.go.

Reference

Remove output type raw_string (#128 - closed)