add --output flag to build-spec command

[edwardmack]

Expected Behavior

Current Behavior

• Currently the build-spec command writes output to the standard output (screen). So the user usually needs to redirect the output.

Possible Solution

• Create a --output flag for build-spec to redirect it's output to the given file.

Steps to Reproduce

Specification

• go version:
• gossamer version:
• gossamer commit tag:
• gossamer commit hash:
• operating system:
• additional links:

Checklist

• I have read CODE_OF_CONDUCT and CONTRIBUTING
• I have provided as much information as possible and necessary
• I am planning to submit a pull request to fix this issue myself