Generate OpenAPI schema from RSpec request specs

There are some gems which generate OpenAPI specs from RSpec request specs. However, they require a special DSL specific to these gems, and we can't reuse existing request specs as they are.Unlike such existing gems, rspec-openapi can generate OpenAPI specs from request specs without requiring any special DSL. Furthermore, rspec-openapi keeps manual modifications when it merges automated changes to OpenAPI specs in case we can't generate everything from request specs.