Review On-Demand Scan YAML Size

as part of the review for !34706 (merged), @fabiopitino highlighted that, unlike other sources which may reference a .gitlab-ci.yml in a repo, a parameter_source stores all of the yaml contents are in full in the db in addition to yaml_variables and options.

as we iterate on-demand scans it's likely the parameter_source contents will grow in size to accommodate passing different environment variable config to dast.

the purpose of this issue is to:

1. ascertain whether or not this redundancy presents an issue
2. find a means of removing/reducing this redundancy