feat: add operation to list all objects

[psankhe28]

Description

Added a GET endpoint that lists the metadata of all the DRS objects.

Fixes #55

Type of change

Please delete options that are not relevant.

• [ ] Bug fix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [ ] Documentation update

Checklist:

• [x] My code follows the style guidelines of this project
• [x] I have performed a self-review of my own code
• [ ] I have commented my code, particularly in hard-to-understand areas
• [x] My changes generate no new warnings
• [ ] I have added tests that prove my fix is effective or that my feature works
• [ ] New and existing unit tests pass locally with my changes
• [ ] I have not reduced the existing code coverage
• [ ] I have added docstrings following the Python style guidelines of this project to all new modules, classes, methods and functions are documented with docstrings following; I have updated any previously existing docstrings, if applicable
• [ ] I have updated any sections of the app's documentation that are affected by the proposed changes, if applicable

[sourcery-ai[bot]]

Reviewer's Guide by Sourcery

This pull request introduces a new feature that adds an endpoint for retrieving all DRS objects. The changes include modifications to the Swagger API definition to add the new endpoint and the implementation of the GetAllObjects function in the server code to handle the retrieval of objects from the database.

File-Level Changes

Files	Changes
drs_filer/api/20200622.383121d.data_repository_service.swagger.yaml drs_filer/ga4gh/drs/server.py	Introduced a new endpoint to list all DRS objects and implemented the corresponding server-side logic to fetch and return these objects from the database.

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