BE: invoice line items

[jacktoncelli]

We implemented the invoice entity in #42, but we still need the invoice line item table. See the design doc here for specifics: https://northeastern-my.sharepoint.com/:w:/g/personal/toncelli_j_northeastern_edu/EcDfJYfhjLZPnUCTmz6MDS8BZzOjXKY-U741GF8FOeGCbQ?e=B2qzUJ

We need the following endpoints:

• POST /quickbooks/invoice/line/bulk: bulk create/update invoice line items
• GET /quickbooks/invoice/line/{id}: get line item by id
• GET /quickbooks/invoice/{id}/lines: get line items from invoice id

Note that the POST operation should be a bulk upsert operation - see how it was implemented for invoices for an example of what we are looking for.

Definition of done:

• Entity created
• Migrations created/locally tested
• Endpoints implemented
• Unit tests written
• OpenAPI docs implemented and manually tested