Integrate Python FileStorageService into EntityService

[tamim-khan]

Ticket link

Closes #81

Implementation description

• Add file name field to MongoDB and Postgres schemas
• Update entity DTO with file name and file fields for create and update operations
• Update entity service for MongoDB and Postgres to call FileStorageService methods
• Update entity REST routes and add get endpoint to retrieve file by UUID
• Add file type validation and update validator to work with multipart form data

Steps to test

Same as #88
REST:
(using Postman)

1. Set the body to form-data and add the body field (as stringified JSON) and file field (postman lets you upload files).

{
    "stringField": "test 1",
    "intField": 1,
    "enumField": "A",
    "stringArrayField": ["test1"],
    "boolField": true
}

(if you want an example)
2. Send the POST request to entities endpoint to create an entity. Verify in firebase console that the file is created.
3. Try creating another entity whose mime type does not belong to text/plain, application/pdf, image/png, image/jpeg, image/gif. Verify that there is a validation error.
4. Try creating another entity without file parameter. Verify that this works (file field is optional).
5. Send a GET request to /entities endpoint. Verify that the file name is returned.
6. Send a GET request to /entities/files/{fileUUID}. Verify that you get the signed URL.
7. Send a PUT request to this entity's endpoint, with a different file. Verify that the file is updated.
8. Send a PUT request to this entity's endpoint, with no file field. Verify that the file is deleted in firebase console.
9. Send a PUT request to this entity's endpoint, with a file field. Verify that a new file UUID is generated and that it appears in firebase console.
10. Send a DELETE request to this entity's endpoint. Verify that the file is deleted in firebase console.
11. Repeat with the other EntityService (mongo or Postgres)

What should reviewers focus on?

• Cases where there is no file
• Update logic

Checklist

• My PR name is descriptive and in imperative tense
• My commit messages are descriptive and in imperative tense. My commits are atomic and trivial commits are squashed or fixup'd into non-trivial commits
• I have run the appropriate linter(s)
• I have requested a review from the PL, as well as other devs who have background knowledge on this PR or who will be building on top of this PR

[sherryhli]

Almost there! A couple more minor issues

[sherryhli]

LGTM, thanks for both of your patience in addressing all the changes! I'll take care of merging this