Add `--assets-links` option

Relates to #66 (closed)

This MR adds the --assets-links option to bin/release-cli create. This allows to pass multiple assets within one option when creating a release.

--assets-links does not replace --assets-link. This means that assets passed through --assets-links should be added on top of what's passed through --assets-link.

Manual testing

bin/release-cli --server-url "http://gitlab.local:3000" --private-token "<private_token>" --project-id 6 create
--description "test asset" --tag-name "v1.5.2" 
--assets-link '{"name":"asset1", "url":"https://gitlab.com/gitlab-org/release-cli"}'
--assets-links '[{"name":"asset2", "url":"https://gitlab.com/gitlab-org"}, {"name":"asset3", "url":"https://gitlab.com/"}]'