Update bulk download endpoint to save holdings UUIDs

Purpose/Overview:
The bulk download API must be able to download UUIDs of both inventory Instances and Holdings

Requirements/Scope:
Users must be allowed to download holdings UUIDs, when searching and filtering from Inventory App
We need to edit the existing API which would return all relevant UUIDs based on query param of instances or holdings
The number of UUIDs can span several thousands to several millions
Given that each UUID is 36 characters, with 1 Million UUIDs the file size would be ~37 MB
For now the largest known data set is UC with ~8Millions Bibs, so file size could be ~300 MB

Requirement
API can return the UUIDs in json response .

Approach:
It can take a query parameter which will decide if instance UUIDs are obtained or holdings UUIDs

Acceptance criteria:

• API is updated, that returns the UUIDs as response
• Unit tests are added