v0.4.0

feat: upgrade Go version and dependencies

- Updated Dockerfile to use Golang 1.26 for building the application.
- Updated go.mod to require Go 1.26 and updated dependencies:
  - github.com/alexflint/go-arg from v1.5.1 to v1.6.1
  - github.com/go-chi/chi/v5 from v5.1.0 to v5.3.0
- Updated go.sum to reflect changes in dependencies.
- Added README.md with detailed usage instructions and configuration examples for the http-rewrite proxy.
- Added TESTING.md outlining the unit test plan for the application.
- Created config_test.go to test YAML configuration parsing.
- Created proxy_handle_test.go to test route registration and error handling in ProxyAPI.
- Created proxy_handler_test.go to test the proxy handler functionality.
- Created proxy_httptarget_test.go to test HTTP target responses.
- Created proxy_mapper_test.go to test variable resolution in the proxy.
- Created wrapper_test.go to test the response writer wrapper functionality.