Quotes: Add opportunity variables to quote content widget merge fields

We would like opportunity-level merge fields, such as the PSA opportunity external identifier, to be available directly within quote content widgets. These fields are currently only available in email templates.

Steps to reproduce:

Open a quote content widget and check the available merge field list. Opportunity fields such as {{quote.opportunity.externalIdentifier}} are not present, even though this same field is documented and works in email templates.

Expected behaviour: Opportunity fields available in email templates are also selectable in the quote content widget variable picker.

Impact: We track work against opportunities in our PSA and want to reference the PSA opportunity ID directly on the quote itself, not just in emails, for internal tracking purposes.