Add unstable_producer_url property to schema and expose in API

[emmambd]

We need a way of marking that a feed is unstable within the schema. This will apply to any URL that includes a time/date or we know from the transit provider that it is not permanent (e.g. it updates monthly).

This should be a field called unstable_producer_url that's added to:

• Catalogs repo
• Public API

default setting -> null

[Alessandro100]

Tasks

• Add unstable_producer_url to catalogs repo
• Add unstable_producer_url to schema
• Sync the catalogs repo unstable_producer_url property with db
• Expose unstable_producer_url in public API