Optionally override codegen script defaults via envvars

Summary:
The codegen helper script, `generate-specs.sh`, is being used to generate code for the FBReactNativeSpec and React-Fabric/rncore pods. The script now supports overriding several defaults by setting the following environment variables:

- SRCS_DIR: Path to JavaScript sources, defaults to $RN_DIR/Libraries/
- LIBRARY_NAME: Defaults to FBReactNativeSpec
- MODULES_OUTPUT_DIR: Defaults to Libraries/$LIBRARY_NAME/$LIBRARY_NAME
- COMPONENTS_LIBRARY_NAME: Defaults to rncore
- COMPONENTS_OUTPUT_DIR: Defaults to ReactCommon/react/renderer/components/$COMPONENTS_LIBRARY_NAME

The CocoaPods codegen integration has been updated to take advantage of these.

**Example CocoaPods usage:**

```
# packages/rn-tester/NativeModuleExample/RNTesterSpecs.podspec
Pod::Spec.new do |s|
  s.name = "RNTesterSpec"
  # ...
  use_react_native_codegen!(s, { :srcs_dir => __dir__, :modules_output_dir => __dir__ })
end
```

Changelog:
[Internal]

Reviewed By: fkgozali

Differential Revision: D25738466

fbshipit-source-id: c68f5a3cd0996283a7af287e992e2f973024f44c