Add --assets-link flag to support direct asset links

Add flag --assets-link='{"name": "Asset1", "url":"https://<domain>/some/location/1", "type": "other", "filepath": "xzy" }'

• It takes a JSON string as a value
• It fails if it's not JSON
• Overrides --assets-links-name and --assets-links-url if --assets-link is defined
• Documentation update done in !50 (merged) merged to this branch

Closes #48 (closed)

Manual tests

• Successful release v1.9.0-rc3 using Docker image from my fork.

• Unsuccessful job when --assets-link` is an invalid JSON https://gitlab.com/jaime/hello/-/jobs/695065061

• Unsuccessful job when there's an API-side validation error https://gitlab.com/jaime/hello/-/jobs/695059640